gh-84995: Run sys.__interactivehook__() on asyncio REPL startup (#20517)

This makes the asyncio REPL (`python -m asyncio`) more usable
and similar to the regular REPL.

This exposes register_readline() as a top-level function in site.py,
but it's intentionally undocumented. 

Co-authored-by: Carol Willing <carolcode@willingconsulting.com>
Co-authored-by: Itamar Oren <itamarost@gmail.com>