Summary:
Upgrading to TypeScript 3.9 so that we can use `// ts-expect-error`, that allows us to basically write unit tests for typings.
Fixed the typescript path in `settings.json`, because otherwise VSCode kept picking the built-in version (3.8.3 atm) of ts when opening the `sonar` folder as workspace.
Reviewed By: passy
Differential Revision: D22255818
fbshipit-source-id: ae54aedb237445f32f1797b4290656dbfb0e528f
8b2d8498e6