TypeScript 4.0 support

[levrik]

Describe the bug

I'm trying to use the new ??= syntax in TypeScript 4.0

This yields Line 53:3: Expected an assignment or function call and instead saw an expression @typescript-eslint/no-unused-expressions as error message.

When I'm silenting this ESLint warning I'm getting Module parse failed: Unexpected token (19:21) from Babel.

I've already tried upgrading all dependencies via yarn upgrade.

Looks like some manual work on create-react-app needs to be done to properly support these new language features.

[levrik]

The general upgrade seems to be tracked by #9515 already.
Will add my findings there.