Add optional `display-name` parameter for custom emoji packs

Related MR: admin-fe!178

Newroma PR: https://codeberg.org/newroma-dev/newroma/pulls/39

This MR adds a display-name parameter for custom emoji packs. Particularly, it allows for pack:tramoji per se to instead return Tramoji whenever a custom emoji's category is asked for. An example here on MastoFE:

As you can see, I also have an emoji pack called custom. When the display-name for an emoji's emoji pack is not available, the category for that emoji instead falls back to pack:{whatever-the-pack-folder-name-is}.

This also has the potential to be useful in the case of Soapbox FE, like seen below with its Emoji Mart picker updated to a newer version: