Deactivate VSCode extension asynchronously

When using HLSL Tools on MacOS, orphaned ShaderTools.LanguageServer
processes will accumulate, wasting memory. This happens after closing
and opening VSCode repeatedly. This does not appear to happen on Windows
in my experience.

microsoft/vscode#35196 (comment)

The above GitHub issue details other developers' experiences with the
same problem. The solution presented by dbaeumer was to return a
`Promise` from an exported `deactivate()`.

https://code.visualstudio.com/api/references/activation-events#Start-up

This is confirmed by the above VSCode docs. Extensions with asynchronous
cleanup processes must return a `Promise` from their `deactivate()`. As
`LanguageServerClient.stop()` returns a `Promise`, we are obligated to
return it when deactivating.

src/ShaderTools.VSCode/src/main.ts

@@ -8,4 +8,8 @@ var sessionManager: SessionManager = undefined;
 export function activate(context: vscode.ExtensionContext): void {
     context.subscriptions.push(sessionManager = new SessionManager());
     sessionManager.start();
+}
+
+export function deactivate(): Promise<void> {
+    return sessionManager.stop();
 }

src/ShaderTools.VSCode/src/session.ts

@@ -35,7 +35,7 @@ export class SessionManager {
         this.startEditorServices();
     }
 
-    public stop() {
+    public stop(): Promise<void> {
         if (this.sessionStatus === SessionStatus.Failed) {
             // Before moving further, clear out the client and process if
             // the process is already dead (i.e. it crashed)
@@ -44,13 +44,17 @@ export class SessionManager {
 
         this.sessionStatus = SessionStatus.Stopping;
 
+        var promise = Promise.resolve();
+
         // Close the language server client
         if (this.languageServerClient !== undefined) {
-            this.languageServerClient.stop();
+            promise = this.languageServerClient.stop();
             this.languageServerClient = undefined;
         }
 
         this.sessionStatus = SessionStatus.NotStarted;
+
+        return promise;
     }
 
     public dispose() : void {