Make the typescript plugins extension point dynamic

[mjbvz]

Part of #66574

Tracks making the typescript plugins extension point dynamic

We should restart the TS server if contributed plugins change.

[mjbvz]

Final part of fix in d73fdb0

[octref]

@mjbvz Ping for verification steps

[mjbvz]

1. In a workspace with tslint setup, such as the vscode source (but without any tslint extension installed)
2. Open file and delete a semicolon. Confirm no tslint alerts
3. Install the tslint extension: https://marketplace.visualstudio.com/items?itemName=ms-vscode.vscode-typescript-tslint-plugin
4. Now the tsserver should restart and tslint should be enabled and alert about the missing semicolons

[octref]

Verified.