tcl-lang.org/v8: split — move tk/v8 to its own PR

Tk depends on tcl-lang.org/v8 (same PR), so its CI couldn't resolve
Tcl's not-yet-published bottle (404). Ship Tcl alone here (it's
self-contained + green); Tk follows in a separate PR once this merges
and Tcl's bottle is published.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: tannevaled