Use non-registered symbols as weak map keys.

[theengineear]

The proposal to allow non-registered symbols (e.g., Symbol('foo')) as keys for weak maps was accepted for 2023. Seems sensible to pivot over to using something like Symbol('__result__') for the opaque references we pass back from our internal template engine.

Currently, x-element creates a new null object (Object.create(null)), which feels dated now that symbols can be used 👌