[hmr] Add an `strict` option in `ReactiveMappingProxy` to always check `_data.__getitem__`

[CNSeniorious000]

This would fix #408, but may not be ideal.

1. Methods __delitem__, __setitem__ (with check_equality=True), and __getitem__ need be updated
2. Even __iter__ might need to call __getitem__ to ensure consistency, or we still can't promise they are in sync
3. Users can manually sync via simply self.update(self._data) if needed

However, without this, direct modifications to __dict__ or globals() still won't be accessible through __getattr__, as noted in #211—this should be considered.

[linear]

PROM-69 [hmr] Add an `strict` option in `ReactiveMappingProxy` to always check `_data.__getitem__`

[CNSeniorious000]

Or we may only add this to Proxy / ReactiveNamespace, which is a semi-private utility just for reactive executing.