extension/src/language: create interactive language client

The interactive language client class extends the language
client by the vscode-languageserver-node. All interactive
related LSP functions and other helper functions are
converted into methods of the interactive client.

The GoLanguageClient now extends InteractiveLanguageClient
so it can access the necessary LSP methods.

For golang/go#76331

Change-Id: I0d3400c56347a14716dde2fcdfa8e049a5cd01d0
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/780200
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Madeline Kalil <mkalil@google.com>

Author: h9jiang

extension/src/context.ts

@@ -4,7 +4,7 @@
  *--------------------------------------------------------*/
 
 import * as vscode from 'vscode';
-import { LanguageClient } from 'vscode-languageclient/node';
+import { InteractiveLanguageClient } from './language/form';
 
 import { LanguageServerConfig, Restart, ServerInfo } from './language/goLanguageServer';
 import { LegacyLanguageService } from './language/registerDefaultProviders';
@@ -14,7 +14,7 @@ import { TelemetryService } from './goTelemetry';
 // They are global so that the server can be easily restarted with
 // new configurations.
 export interface GoExtensionContext {
-	languageClient?: LanguageClient;
+	languageClient?: InteractiveLanguageClient;
 	legacyLanguageService?: LegacyLanguageService;
 	latestConfig?: LanguageServerConfig;
 	telemetryService?: TelemetryService;