feat: show week numbers

[NicolaiSoeborg]

Controllable by cosmic-settings with this commit:
NicolaiSoeborg/cosmic-settings@de3be06 / pop-os/cosmic-settings#2028

Screenshots (dark / light theme):

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[jklgrasso]

With week numbers enabled, the last day in the calendar is cut off. Everything else looks good

[jacobgkau]

I also see the last weekday cut off on this branch, as shown in the PR description's screenshot. Just to be clear (with a formal review), that would need to be fixed for this to move forward.