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.

lua/ale/lsp.lua

@@ -20,7 +20,15 @@ module.start = function(config)
         end
     }
 
-    config.on_init = function(_, _)
+    config.on_init = function(client, _)
+        -- Tell ALE about server capabilities as soon as we can.
+        -- This will inform ALE commands what can be done with each server,
+        -- such as "go to definition" support, etc.
+        vim.fn["ale#lsp#UpdateCapabilities"](
+            config.name,
+            client.server_capabilities
+        )
+
         -- Neovim calls `on_init` before marking a client as active, meaning
         -- we can't get a client via get_client_by_id until after `on_init` is
         -- called. By deferring execution of calling the init callbacks we
@@ -55,6 +63,7 @@ module.send_message = function(args)
     local client = vim.lsp.get_client_by_id(args.client_id)
 
     if args.is_notification then
+        -- For notifications we send a request and expect no direct response.
         local success = client.notify(args.method, args.params)
 
         if success then
@@ -64,10 +73,23 @@ module.send_message = function(args)
         return 0
     end
 
-    -- NOTE: We aren't yet handling reponses to requests properly!
-    -- NOTE: There is a fourth argument for a bufnr here, and it's not
-    --       clear what that argument is for or why we need it.
-    local success, request_id = client.request(args.method, args.params)
+    local success, request_id
+
+    -- For request we send a request and handle the response.
+    --
+    -- We set the bufnr to -1 to prevent Neovim from flushing anything, as ALE
+    -- already flushes changes to files before sending requests.
+    success, request_id = client.request(
+        args.method,
+        args.params,
+        function(_, result, _, _)
+            vim.fn["ale#lsp#HandleResponse"](client.name, {
+                id = request_id,
+                result = result,
+            })
+        end,
+        -1
+    )
 
     if success then
         return request_id