Handle other LSP capabilities via ALE

Save capabilities from language servers ALE connects to via Neovim's LSP
client and handle responses to requests ALE sends to language servers.
This enables ALE to work with Neovim's language client for its commands
while also letting users directly use the connected clients for native
Neovim keybinds and so on.

autoload/ale/lsp.vim

@@ -195,101 +195,107 @@ endfunction
 
 " Update capabilities from the server, so we know which features the server
 " supports.
-function! s:UpdateCapabilities(conn, capabilities) abort
+function! ale#lsp#UpdateCapabilities(conn_id, capabilities) abort
+    let l:conn = get(s:connections, a:conn_id, {})
+
+    if empty(l:conn)
+        return
+    endif
+
     if type(a:capabilities) isnot v:t_dict
         return
     endif
 
     if get(a:capabilities, 'hoverProvider') is v:true
-        let a:conn.capabilities.hover = 1
+        let l:conn.capabilities.hover = 1
     endif
 
     if type(get(a:capabilities, 'hoverProvider')) is v:t_dict
-        let a:conn.capabilities.hover = 1
+        let l:conn.capabilities.hover = 1
     endif
 
     if get(a:capabilities, 'referencesProvider') is v:true
-        let a:conn.capabilities.references = 1
+        let l:conn.capabilities.references = 1
     endif
 
     if type(get(a:capabilities, 'referencesProvider')) is v:t_dict
-        let a:conn.capabilities.references = 1
+        let l:conn.capabilities.references = 1
     endif
 
     if get(a:capabilities, 'renameProvider') is v:true
-        let a:conn.capabilities.rename = 1
+        let l:conn.capabilities.rename = 1
     endif
 
     if type(get(a:capabilities, 'renameProvider')) is v:t_dict
-        let a:conn.capabilities.rename = 1
+        let l:conn.capabilities.rename = 1
     endif
 
     if get(a:capabilities, 'codeActionProvider') is v:true
-        let a:conn.capabilities.code_actions = 1
+        let l:conn.capabilities.code_actions = 1
     endif
 
     if type(get(a:capabilities, 'codeActionProvider')) is v:t_dict
-        let a:conn.capabilities.code_actions = 1
+        let l:conn.capabilities.code_actions = 1
     endif
 
     if !empty(get(a:capabilities, 'completionProvider'))
-        let a:conn.capabilities.completion = 1
+        let l:conn.capabilities.completion = 1
     endif
 
     if type(get(a:capabilities, 'completionProvider')) is v:t_dict
         let l:chars = get(a:capabilities.completionProvider, 'triggerCharacters')
 
         if type(l:chars) is v:t_list
-            let a:conn.capabilities.completion_trigger_characters = l:chars
+            let l:conn.capabilities.completion_trigger_characters = l:chars
         endif
     endif
 
     if get(a:capabilities, 'definitionProvider') is v:true
-        let a:conn.capabilities.definition = 1
+        let l:conn.capabilities.definition = 1
     endif
 
     if type(get(a:capabilities, 'definitionProvider')) is v:t_dict
-        let a:conn.capabilities.definition = 1
+        let l:conn.capabilities.definition = 1
     endif
 
     if get(a:capabilities, 'typeDefinitionProvider') is v:true
-        let a:conn.capabilities.typeDefinition = 1
+        let l:conn.capabilities.typeDefinition = 1
     endif
 
     if type(get(a:capabilities, 'typeDefinitionProvider')) is v:t_dict
-        let a:conn.capabilities.typeDefinition = 1
+        let l:conn.capabilities.typeDefinition = 1
     endif
 
     if get(a:capabilities, 'implementationProvider') is v:true
-        let a:conn.capabilities.implementation = 1
+        let l:conn.capabilities.implementation = 1
     endif
 
     if type(get(a:capabilities, 'implementationProvider')) is v:t_dict
-        let a:conn.capabilities.implementation = 1
+        let l:conn.capabilities.implementation = 1
     endif
 
     if get(a:capabilities, 'workspaceSymbolProvider') is v:true
-        let a:conn.capabilities.symbol_search = 1
+        let l:conn.capabilities.symbol_search = 1
     endif
 
     if type(get(a:capabilities, 'workspaceSymbolProvider')) is v:t_dict
-        let a:conn.capabilities.symbol_search = 1
+        let l:conn.capabilities.symbol_search = 1
     endif
 
     if type(get(a:capabilities, 'textDocumentSync')) is v:t_dict
         let l:syncOptions = get(a:capabilities, 'textDocumentSync')
 
         if get(l:syncOptions, 'save') is v:true
-            let a:conn.capabilities.did_save = 1
+            let l:conn.capabilities.did_save = 1
         endif
 
         if type(get(l:syncOptions, 'save')) is v:t_dict
-            let a:conn.capabilities.did_save = 1
+            let l:conn.capabilities.did_save = 1
 
             let l:saveOptions = get(l:syncOptions, 'save')
 
             if get(l:saveOptions, 'includeText') is v:true
-                let a:conn.capabilities.includeText = 1
+                let l:conn.capabilities.includeText = 1
             endif
         endif
     endif
@@ -334,7 +340,7 @@ function! ale#lsp#HandleInitResponse(conn, response) abort
         let a:conn.initialized = 1
     elseif type(get(a:response, 'result')) is v:t_dict
     \&& has_key(a:response.result, 'capabilities')
-        call s:UpdateCapabilities(a:conn, a:response.result.capabilities)
+        call ale#lsp#UpdateCapabilities(a:conn.id, a:response.result.capabilities)
 
         let a:conn.initialized = 1
     endif
@@ -385,6 +391,20 @@ function! ale#lsp#HandleMessage(conn_id, message) abort
     endif
 endfunction
 
+" Handle a JSON response from a language server.
+" This is called from Lua for integration with Neovim's LSP API.
+function! ale#lsp#HandleResponse(conn_id, response) abort
+    let l:conn = get(s:connections, a:conn_id, {})
+
+    if empty(l:conn)
+        return
+    endif
+
+    for l:Callback in l:conn.callback_list
+        call ale#util#GetFunction(l:Callback)(a:conn_id, a:response)
+    endfor
+endfunction
+
 " Given a connection ID, mark it as a tsserver connection, so it will be
 " handled that way.
 function! ale#lsp#MarkConnectionAsTsserver(conn_id) abort