Imports the runtime from `React.unstable_ForgetRuntime` rather than from a
separate module. The hope is that any code that gets transformed already has a
dependency on React anyway, so we can avoid adding a new dependency that other
systems don't know about.
While here, i also cleaned up the `guardThrows` flag (we still parse it if
present and warn, rather than throwing, to make it easier to adopt the latest
version in various places).
This is a new module that holds:
- the `useMemoCache` stub (hopefully to be deleted next week)
- various helpers that can be imported by the compiler, e.g. the dispatcher
guard `$startLazy`
- skipped the implementation of `makeReadOnly` for now as there's already
multiple copies and I wanted to avoid typescript in this file for now to make
the build easier (i.e. no build)