Automatically generate plugin activation events

[msujew]

What it does

Closes #12161

When loading a plugin, this change automatically deduces some of its activation events from the plugin manifest.

This change also adds a few more entries to the manifest types, which were required for this to work correctly.

I also did a drive-by cleanup of the scanner-theia file. A lot of the non-null assertions were no longer required due to advancements of TypeScript type checker.

How to test

1. Download and install this test extension. The extension is the default hello world extension from vscode, with all of its activationEvents removed.
2. Run the Hello World command
3. Assert that the extension starts and a notification is displayed.

Review checklist

• 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

[vince-fugnitto]

The changes look good and well for me 👍

• confirmed that the "hello world" plugin is correctly activated
• confirmed that existing plugins (ex: builtins) are still activated properly