Constants: add numeric css class

[danirabbit]

We're currently manually using pango for tnum features in a number of places: https://github.com/search?q=org%3Aelementary+tnum+language%3AVala&type=code&l=Vala

Adds numeric style class which has precedent in Adwaita: https://gitlab.gnome.org/GNOME/libadwaita/-/merge_requests/217

Created a new CssClass namespace in line with: #741

[jeremypw]

Looks good and works as expected. Note that it depends on itself to compile the demo so you have to install without the demo option first and then again with the demo option. Presumably this will not affect the final package?

[danirabbit]

Yeah CI builds it fine and we haven't had issues with since breaking package builds before that I'm aware of