[vscode] Stub activeStackItem and related change event in debug namespace

[rschnekenbu]

What it does

Stubs the new API on debug namespace about activeStackItem and event when this value is changed.

fixes #13758

contributed on behalf of STMicroelectronics

How to test

1. Launch master theia browser example, install the following extension
   • src: debug-extension-sample-0.0.1-src.zip
   • zip vsix: debug-extension-sample-0.0.1.zip
2. When starting the example, a message will be logged in the debug console: Activating extension 'debug-extension-sample' failed: TypeError: vscode.debug.onDidChangeActiveStackItem is not a function
3. With the PR and the extension, no message is logged in the debug console. The extension is supposed to log in the console in case of selection change in the debug views, but obviously does nothing currently due to stub implementation

Follow-ups

The current stub may be replaced by the real implementation. See #13846

Review checklist

• [x] As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines