gh-143990: Allow tkinter.font.Font to wrap a font description (GH-152025)

gh-143990: Allow tkinter.font.Font to wrap a font description (GH-152025)

With exists=True and no name, Font now wraps the font description as is,
without creating a new named font, so that it is used without loss of
precision by actual(), measure() and metrics().  Its name attribute is then
the description rather than a string.  Keyword options now override the
corresponding settings of the given font instead of being ignored.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>