Use a unified script, to reduce code duplication and in preparation
for doing a similar thing with styles and filters.  The new script
also uses a bit more modern Python APIs (e.g., pathlib).

Unlike the previous scripts, it does not replace replace CRLF with LF
because Git should do that itself.

This change also adds a script to regenerate the list of CSS properties
from the W3C source if needed.

and use it in the "duplicate filenames" checker.

Use -m build instead of setup.py.