gh-113317: Clean up Argument Clinic global namespace (#113414)

Split up clinic.py by establishing libclinic as a support package for
Argument Clinic. Get rid of clinic.py globals by either making them
class members, or by putting them into libclinic.

- Move INCLUDE_COMMENT_COLUMN to BlockPrinter
- Move NO_VARARG to CLanguage
- Move formatting helpers to libclinic
- Move some constants to libclinic (and annotate them as Final)