Fix debugger to start language client when necessary (#4125)

Rather than just throwing an error message. Also don't activate on
`onDebugInitialConfigurations` so that the extension doesn't startup
unnecessarily.

Author: andyleejordan

package.json

@@ -26,7 +26,6 @@
     "url": "https://github.com/PowerShell/vscode-powershell.git"
   },
   "activationEvents": [
-    "onDebugInitialConfigurations",
     "onDebugResolve:PowerShell",
     "onLanguage:powershell",
     "onCommand:PowerShell.NewProjectFromTemplate",

src/features/DebugSession.ts

@@ -153,13 +153,9 @@ export class DebugSessionFeature extends LanguageClientConsumer
         _folder: WorkspaceFolder | undefined,
         config: DebugConfiguration,
         _token?: CancellationToken): Promise<DebugConfiguration> {
-        // Make sure there is a session running before attempting to debug/run a program
-        // TODO: Perhaps this should just wait until it's running or aborted.
+
         if (this.sessionManager.getSessionStatus() !== SessionStatus.Running) {
-            const msg = "Cannot debug or run a PowerShell script until the PowerShell session has started. " +
-                "Wait for the PowerShell session to finish starting and try again.";
-            vscode.window.showWarningMessage(msg);
-            return undefined;
+            await this.sessionManager.start();
         }
 
         // Starting a debug session can be done when there is no document open e.g. attach to PS host process