Add ALEGoToImplementation (#4160)

* Add go to implementation

* Add test cases for GoToImplementation

* Add documentation for GoToImplementation

test/lsp/test_lsp_client_messages.vader

@@ -175,6 +175,20 @@ Execute(ale#lsp#message#TypeDefinition() should return correct messages):
   \ ],
   \ ale#lsp#message#TypeDefinition(bufnr(''), 12, 34)
 
+Execute(ale#lsp#message#Implementation() should return correct messages):
+  AssertEqual
+  \ [
+  \   0,
+  \   'textDocument/implementation',
+  \   {
+  \     'textDocument': {
+  \         'uri': ale#path#ToFileURI(g:dir . '/foo/bar.ts'),
+  \     },
+  \     'position': {'line': 11, 'character': 33},
+  \   }
+  \ ],
+  \ ale#lsp#message#Implementation(bufnr(''), 12, 34)
+
 Execute(ale#lsp#message#References() should return correct messages):
   AssertEqual
   \ [
@@ -335,6 +349,19 @@ Execute(ale#lsp#tsserver_message#TypeDefinition() should return correct messages
   \ ],
   \ ale#lsp#tsserver_message#TypeDefinition(bufnr(''), 347, 12)
 
+Execute(ale#lsp#tsserver_message#Implementation() should return correct messages):
+  AssertEqual
+  \ [
+  \   0,
+  \   'ts@implementation',
+  \   {
+  \     'file': ale#path#Simplify(g:dir . '/foo/bar.ts'),
+  \     'line': 347,
+  \     'offset': 12,
+  \   }
+  \ ],
+  \ ale#lsp#tsserver_message#Implementation(bufnr(''), 347, 12)
+
 Execute(ale#lsp#tsserver_message#References() should return correct messages):
   AssertEqual
   \ [

test/lsp/test_lsp_startup.vader

@@ -189,6 +189,10 @@ Before:
       \           'typeDefinition': {
       \             'dynamicRegistration': v:false,
       \           },
+      \           'implementation': {
+      \             'dynamicRegistration': v:false,
+      \             'linkSupport': v:false,
+      \           },
       \           'publishDiagnostics': {
       \             'relatedInformation': v:true,
       \           },

test/lsp/test_other_initialize_message_handling.vader

@@ -182,6 +182,7 @@ Execute(Capabilities should be enabled when send as Dictionaries):
   \     },
   \     'definitionProvider': {},
   \     'typeDefinitionProvider': {},
+  \     'implementationProvider': {},
   \     'experimental': {},
   \     'documentHighlightProvider': v:true,
   \     'workspaceSymbolProvider': {}
@@ -198,6 +199,7 @@ Execute(Capabilities should be enabled when send as Dictionaries):
   \   'hover': 1,
   \   'definition': 1,
   \   'typeDefinition': 1,
+  \   'implementation': 1,
   \   'symbol_search': 1,
   \   'rename': 1,
   \   'code_actions': 1,