Porting parser_function_count.py from compat to core/scripts
- Added parsefunctioncount.py in core/scripts/ as part of Pywikibot:Compat to Core migration.
- use SingleSiteBot, ExistingPageBot, NoRedirectPageBot classes
- use collections.Counter to hold the result
- Use magicwords for variable declaration via siteinfo.
- Use doc_subpages infrastructure of core
Bug: T66878
Change-Id: I28aafa2aa2928b9585fc825523dfc17fb808e4f9