TypeScript 3.9 `lib.d.ts` updates

[DanielRosenwasser]

• Ensure that PRs are merged in.
• Sync with https://github.com/Microsoft/TSJS-lib-generator/

[ExE-Boss]

It’d probably be a good idea to do microsoft/TypeScript-DOM-lib-generator#826 and #37510.

[sandersn]

This is done now.