Normative: Permit Symbols as WeakMap keys (#2777)

- Proposal: https://github.com/tc39/proposal-symbols-as-weakmap-keys
- Also allows Symbols in WeakSet, WeakRef, and FinalizationRegistry
- Adds new AOs `CanBeHeldWeakly`, `KeyForSymbol`
- Registered Symbols can not be held weakly

Closes #1194

Co-authored-by: Ashley Claymore <aclaymore@bloomberg.net>
Co-authored-by: Daniel Ehrenberg <dehrenberg@bloomberg.net>
Co-authored-by: Leo Balter <leonardo.balter@gmail.com>
Co-authored-by: Mathieu Hofman <86499+mhofman@users.noreply.github.com>
Co-authored-by: Richard Gibson <richard.gibson@gmail.com>
Co-authored-by: Jordan Harband <ljharb@gmail.com>
Co-authored-by: Shu-yu Guo <syg@chromium.org>
Co-authored-by: Michael Dyck <jmdyck@ibiblio.org>
Co-authored-by: Michael Ficarra <mficarra@shapesecurity.com>