Add metadata filtering

If the symbols could have more metadata associated with them, then the filtering functionality of this library could be pretty cool. I'd like to see the Greek alphabet be grouped together, for instance.

If KaTeX doesn't want support this, then maybe that extra metadata could be added into this library.

Start by reviewing the library's existing symbol-list and filtering behavior, then determine whether metadata should come from KaTeX or be maintained here. Define how symbols such as the Greek alphabet are grouped and what filtering results should look like; the issue is done when metadata-backed filtering works consistently.

Written by the indexing model from the issue text.