During development of a component, it was found that re-renders of an

Ink.KeyboardShortcut was causing collisions in the registry, since

there was never an opportunity to unregister the old shortcut. This

situation could easily happen in any number of cases, and a solution was

needed.
KeyboardShortcutRegistry now accepts a boundObj during registration,

which is then tracked using a WeakRef and a FinalizationRegistry.

These work to proactively unregister shortcuts for bound objects when

they're garbage-collected, and also to ignore registered-but-expired

entries when registering or returning results.
As part of this, I've redone how we fetch the full list of shortcuts.

Instead of building and returning an array, we now provide an iterator,

which is pretty widely-supported now. This lets us iterate over the

registered shortcuts, filtering out any from results as we go.
To faciliate this, and to ensure consistency in registration orders, we

now use a Map instead of a plain object for registration.
And last, I realized we didn't have a symbol entry on macOS for "Enter".

This isn't related to the rest of the change, but I'm sneaking it in.

Unit tests pass.
Verified this resolved the issue with colliding shortcuts when elements

were garbage-collected.