fix: cleanup _idmap of $ZodRegistry

[safareli]

No description provided.

[safareli]

shouldn't _idmap: Map<string, Schema> = new Map(); be WeakMap too? otherwise presense of id in meta results in strong pointer of Schema to be created which means it will not be removed from the _map ever even if there are no strong pointers to Schema from outside of this class thus creating mem leak potential.

[colinhacks]

You're correct. Unfortunately registries need to be iterable for z.toJSONSchema() as implemented (this is why CI is failing), and the fact that it's using WeakRef for the schema map is a bit of a vestige of an older implementation. I think I'll be switching to a plain Map and adding a .clear() method. It's not ideal, but honestly I think a true WeakMap implementation would also cause a lot of mysterious issues for people who don't understand how they work. Regardless, merging this fix 👍 Thank you.