Fix TypeScript version target for types

[taion]

See e.g. https://unpkg.com/browse/react-overlays@3.1.3/esm/Overlay.d.ts. This uses Omit, which doesn't work for older TS versions.

[kosmiq]

What's the minimum version we should require and expect people to have? 3.5 that added omit was released in June 2019, is that too new?

[taion]

Huh, is there anything else that might show up in our .d.ts files? The people reporting e.g. issues in CI but not locally esp. around <Modal> are making me suspicious.