Basically, we detect if a glyph is a variation selector if its `id` is 3
(i.e. a whitespace character) and if it comes from an emoji font (since
variation selectors are only used for emoji glyphs).

- Fixes https://github.com/zed-industries/zed/issues/11703 and
https://github.com/zed-industries/zed/issues/12022

Release Notes:

- N/A