Add support for font features and variants

[mattt]

Overview: What does this pull request change?

This PR adds support for font features and font variants in text elements rendered with Pango. It relies on changes in ManimCommunity/ManimPango#121. When using versions of ManimPango that don't support font features or variants (like, the latest release) this has no effect, thanks to wrapping attr setters using with suppress(Exception).

Motivation and Explanation: Why and how do your changes improve the library?

Looking at the CountingScene example in the docs, I noticed that the layout shifting horizontally as the numbers were incrementing. I went to update the example to use tabular instead of proportional figures, but didn't see a way to do that.

Digging in further, I found a way to do this in LaTeX (I think... PR forthcoming), but not for SVG text. This led me down a rabbit hole that resulted in ManimCommunity/ManimPango#121 and now this PR.

Links to added or changed documentation pages

Further Information and Comments

Reviewer Checklist

• The PR title is descriptive enough for the changelog, and the PR is labeled correctly
• If applicable: newly added non-private functions and classes have a docstring including a short summary and a PARAMETERS section
• If applicable: newly added functions and classes are tested