Expose LegacyHidden type and disable <div hidden /> API in new fork (#18891)

* Expose LegacyHidden type

I will use this internally at Facebook to migrate away from
<div hidden />. The end goal is to migrate to the Offscreen type, but
that has different semantics. This is an incremental step.

* Disable <div hidden /> API in new fork

Migrates to the unstable_LegacyHidden type instead. The old fork does
not support the new component type, so I updated the tests to use an
indirection that picks the correct API. I will remove this once the
LegacyHidden (and/or Offscreen) type has landed in both implementations.

* Add gated warning for `<div hidden />` API

Only exists so we can detect callers in www and migrate them to the new
API. Should not visible to anyone outside React Core team.