Add a lsp_config_callback linter option

This is the callback-based variant of the existing `lsp_config` linter
option. It serves the same purpose but can be used when more complicated
processing is needed.

`lsp_config` and `lsp_config_callback` are mutually exclusive options;
if both an given, a linter preprocessing error will be raised.

The runtime logic has been wrapped in `ale#lsp_linter#GetConfig` for
convenience, similar to `ale#lsp_linter#GetOptions`.

This also adds documentation and an `AssertLSPConfig` test function for
completeness.

autoload/ale/assert.vim

@@ -85,6 +85,14 @@ function! ale#assert#LSPOptions(expected_options) abort
     AssertEqual a:expected_options, l:initialization_options
 endfunction
 
+function! ale#assert#LSPConfig(expected_config) abort
+    let l:buffer = bufnr('')
+    let l:linter = s:GetLinter()
+    let l:config = ale#lsp_linter#GetConfig(l:buffer, l:linter)
+
+    AssertEqual a:expected_config, l:config
+endfunction
+
 function! ale#assert#LSPLanguage(expected_language) abort
     let l:buffer = bufnr('')
     let l:linter = s:GetLinter()
@@ -147,6 +155,7 @@ function! ale#assert#SetUpLinterTest(filetype, name) abort
     command! -nargs=+ AssertLinter :call ale#assert#Linter(<args>)
     command! -nargs=0 AssertLinterNotExecuted :call ale#assert#LinterNotExecuted()
     command! -nargs=+ AssertLSPOptions :call ale#assert#LSPOptions(<args>)
+    command! -nargs=+ AssertLSPConfig :call ale#assert#LSPConfig(<args>)
     command! -nargs=+ AssertLSPLanguage :call ale#assert#LSPLanguage(<args>)
     command! -nargs=+ AssertLSPProject :call ale#assert#LSPProject(<args>)
     command! -nargs=+ AssertLSPAddress :call ale#assert#LSPAddress(<args>)
@@ -172,6 +181,10 @@ function! ale#assert#TearDownLinterTest() abort
         delcommand AssertLSPOptions
     endif
 
+    if exists(':AssertLSPConfig')
+        delcommand AssertLSPConfig
+    endif
+
     if exists(':AssertLSPLanguage')
         delcommand AssertLSPLanguage
     endif

autoload/ale/linter.vim

@@ -257,7 +257,17 @@ function! ale#linter#PreProcess(filetype, linter) abort
             let l:obj.initialization_options = a:linter.initialization_options
         endif
 
-        if has_key(a:linter, 'lsp_config')
+        if has_key(a:linter, 'lsp_config_callback')
+            if has_key(a:linter, 'lsp_config')
+                throw 'Only one of `lsp_config` or `lsp_config_callback` should be set'
+            endif
+
+            let l:obj.lsp_config_callback = a:linter.lsp_config_callback
+
+            if !s:IsCallback(l:obj.lsp_config_callback)
+                throw '`lsp_config_callback` must be a callback if defined'
+            endif
+        elseif has_key(a:linter, 'lsp_config')
             if type(a:linter.lsp_config) isnot v:t_dict
                 throw '`lsp_config` must be a Dictionary'
             endif

autoload/ale/lsp_linter.vim

@@ -140,6 +140,18 @@ function! ale#lsp_linter#GetOptions(buffer, linter) abort
     return l:initialization_options
 endfunction
 
+function! ale#lsp_linter#GetConfig(buffer, linter) abort
+    let l:config = {}
+
+    if has_key(a:linter, 'lsp_config_callback')
+        let l:config = ale#util#GetFunction(a:linter.lsp_config_callback)(a:buffer)
+    elseif has_key(a:linter, 'lsp_config')
+        let l:config = a:linter.lsp_config
+    endif
+
+    return l:config
+endfunction
+
 " Given a buffer, an LSP linter, start up an LSP linter and get ready to
 " receive messages for the document.
 function! ale#lsp_linter#StartLSP(buffer, linter) abort
@@ -188,14 +200,16 @@ function! ale#lsp_linter#StartLSP(buffer, linter) abort
         call ale#lsp#MarkConnectionAsTsserver(l:conn_id)
     endif
 
-    let l:language_id = ale#util#GetFunction(a:linter.language_callback)(a:buffer)
+    let l:config = ale#lsp_linter#GetConfig(a:buffer, a:linter)
 
-    if !empty(get(a:linter, 'lsp_config'))
+    if !empty(l:config)
         " set LSP configuration options (workspace/didChangeConfiguration)
-        let l:config_message = ale#lsp#message#DidChangeConfiguration(a:buffer, a:linter.lsp_config)
+        let l:config_message = ale#lsp#message#DidChangeConfiguration(a:buffer, l:config)
         call ale#lsp#Send(l:conn_id, l:config_message)
     endif
 
+    let l:language_id = ale#util#GetFunction(a:linter.language_callback)(a:buffer)
+
     let l:details = {
     \   'buffer': a:buffer,
     \   'connection_id': l:conn_id,