Pick up TS 4.2.1

[mjbvz]

Tracks picking up the final release of TypeScript 4.2.

You can read more about the release here:

• TypeScript 4.2-beta blog post

• TypeScript 4.2 iteration plan

[mjbvz]

Nightly picked up with 925d923

[rzhao271]

I still see nightly here: https://github.com/microsoft/vscode/blob/main/package.json#L194
The TS dependencies in test/automation, test/integration/browser, test/smoke are also on v3.7.5.
Labelling as verification found, since I'm not sure if this is intentional.