Improve handling of specified font name

[behackl]

Currently our documentation build throws several warnings like

[10/22/23 18:12:47] WARNING  Font monospace not in [...]
[10/22/23 18:14:58] WARNING  Font sans not in [...]
[10/22/23 18:15:00] WARNING  Font sans-serif not in [...]

See https://readthedocs.org/api/v2/build/22307289.txt for the complete log of a recent build.

There are two issues here:

• The font sans-serif should probably also be referred to as sans.
• Miscapitalized fonts are currently not registered correctly: in the examples above, Monospace and Sans are actually included by the list returned by manimpango.list_fonts(). I think it would be a good idea to automatically correct this user input error: if the specified font is not included in the list, manim should check whether the capitalized version is there -- and if so, use that value instead.

[staghado]

Can I take a stab at this?

[behackl]

Can I take a stab at this?

Sure, much appreciated! Feel free to ask questions here or in the development-chat in our Discord -- but otherwise just submit a PR with your proposed changes.