Housekeeping & improvements, part 2

[akx]

This PR builds on #23:

• It makes the TypeScript configuration a bit stricter, for cleaner code. 🧹
• It gets rid of the separate e.ts/e.native.ts modules; it didn't feel necessary to have a different .native.ts file when you can detect ReactNativeness just as easily. At the same, e was renamed to useIsomorphicLayoutEffect (which is a common name for the idiom), for readability.
• Speaking of readability, this PR humanizes some of the names used by the external interface, so they're easier to understand for end-users of the library.
• It slightly improves the types for less repetition; for instance, React.SetStateAction is a dead ringer for the interface used for the setter in this library.
• It removes an apparently (?) unused ac parameter that didn't match the actually returned type anyway.
• It switches to optional chaining wherever suitable.
  • However, considering the coverage for optional chaining is only 90% at present, this switches the esbuild target to es2019.

All things considered, the files in dist/ are still smaller than in main (448e745); 2023 bytes there, 1590 bytes here. 😇

[RichardLindhout]

Thanks, looks good!