Closes #8265.

This implements support for filtering symbols by math class in symbol search by adding class:<class> as a search term, where <class> is case-insensitive and can be:

• none, matching symbols without a math class;
• a Unicode math class or class name¹ (i.e., the one-letter form, like X, or the full name, like Special), matching symbols with this math class.²

Alternatively, one can filter out symbols by adding a minus sign before the filter (e.g., -class:none displays all symbols that do have a math class). Note that this operates on the default math class assigned to each symbol by Typst. Notably, Typst does not fully follow UTR #25 as it overrides the default math class for some symbols.³

This is currently a draft because I am unsure whether this is the right way to expose this feature. This also needs to be documented, which is done is perhaps the most ugly way right now. Any input is welcome!

Also, I implemented this in a way that makes it easy to define further filters in the future.

The first commit is a somewhat-related minor fix. Since Codex doesn't assign names to Glyph_Part symbols, and likely never will, the bug is unlikely to ever surface anyway.