Freeze loaded manifests (#64313)
Manifests in the runtime should be treated as immutable objects to
ensure that side effects aren't created that depend on the mutability of
these shared objects. Instead, mutable references should be used in
places where this is desirable.
This also introduces the new `DeepReadonly` utility type, which when
paired with existing manifest types, will modify every field to be read
only, ensuring that the type system will help catch accidental
modifications to these loaded manifest values as well.
Future work could eliminate these types by modifying the manifest types
themselves to be read only, only allowing code that generated them to be
writable.
Closes NEXT-3069
