Add nothreads feature tag to signify lack of THREADS_ENABLED

[mihe]

As talked about in #93556.

This adds a new feature tag to OS::has_feature called nothreads, which complements the existing threads feature tag that's meant to signify whether the particular builds has THREADS_ENABLED defined or not, meaning whether it's been built with threads=yes.

This can help define things that should only apply to single-threaded builds (likely web builds), like project settings overrides or specific GDExtension binaries.

[RandomShaper]

Makes sense. Even if in a future release we add some syntax to negate features, we could keep this hardcoded one as a backcompat.

[akien-mga]

Thanks!