Split & remove extension_parallel (#8983)

As per plan in #8932, we want to be able to include thread_parallel.h to build
libraries that are capable of parallelization, but don't require it. So, we
move the header to ExecuTorch core and add a fallback implementation (with
tests!) of parallel_for that just does a regular for loop. Then, targets
that link extension_threadpool will get parallelization automagically.

This PR doesn't add any optionally-parallelized code; that will be in the next
PR.