Relax `react` requirement to v16.8-18

[opiation]

This react-mathquill package only uses the following exports from the peer React package:

• useEffect
• useLayoutEffect
• useRef
• useState (only used in examples/, not implementation)

These exports are all available in React as of v16.8.0 when hooks were first introduced and they have behaved fairly consistently since then. This seems like sufficient reason to relax the version requirements for the react peer dependency.

An example use case that benefits from this widened support allows one to upgrade react from v16.8.0 to v18.2.0 without having to update this package at the same time, shrinking the surface area of change that needs to happen all at once.

[opiation]

@viktorstrate, and chance of this being approved and merged? Or is there some reason why this is undesirable?