[release] src/goLanguageServer: block until the language client is ready

According to https://github.com/microsoft/vscode-languageserver-node#breaking-changes,
we need to wait until the client is actually ready before attempting to make any
request including shutdown.

Change-Id: Icfa4c9069ad448156794a7d14ba9036a14bdfd55
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/288372
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
(cherry picked from commit 55b2284)
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/289232

Author: hyangah

src/goLanguageServer.ts

@@ -242,6 +242,7 @@ async function startLanguageServer(ctx: vscode.ExtensionContext, config: Languag
 	languageServerDisposable = languageClient.start();
 	totalStartCount++;
 	ctx.subscriptions.push(languageServerDisposable);
+	await languageClient.onReady();
 	return true;
 }