upgrade to typescript 3.9.2

[akosyakov]

What it does

• 3.9 release is available https://devblogs.microsoft.com/typescript/announcing-typescript-3-9/

How to test

• build and try
  • there can be some issue with typescript extension but they are related to latest built-in extension support: investigate integration tests log output against latest built-in #7800
• integration tests should pass

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 to me 👍
The tests all pass and are green, and I also performed a quick sanity check (application starts, go-to-definition works).