ci: Use python to extract JS snippets from style guide

ci: Use python to extract JS snippets from style guide

The job that lints code snippets in the style guide only runs
when either the documentation or the linting rules are changed.

Because of that it went unnoticed that the awk tool it uses to
extract the snippets is no longer available since the CI image
was rebased to F42.

Given that awk is awkward anyway, use the opportunity to rewrite the
snippet extraction in python instead of adding awk back to the image.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3694>