Normative: Permit Symbols as WeakMap keys

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

Closes tc39#1194

Co-authored-by: Daniel Ehrenberg <[email protected]>
Co-authored-by: Leo Balter <[email protected]>
Co-authored-by: Mathieu Hofman <[email protected]>
Co-authored-by: Richard Gibson <[email protected]>
Co-authored-by: Jordan Harband <[email protected]>
Co-authored-by: Shu-yu Guo <[email protected]>
Co-authored-by: Michael Dyck <[email protected]>
Co-authored-by: Michael Ficarra <[email protected]>