docs: rewrite and expand README

- fix grammar/typos and reword awkward sentences
- restructure: Features, Installation, Quick start, How it works,
  Controlling what gets marshalled, Security, API, Limitations
- document previously-undocumented APIs (evalModule, runtime
  limits/stats, registerAll/unregister signatures)
- cover syncEnabled, marshalByReference, AsyncArena, bigint, and
  defineProperty synchronization
- fix a duplicate unregisterAll heading (was actually registerAll)

Author: rot1024