Create adapters for Emoji

We currently have an Emoji plugin defined, but we have no adapter for it. We should translate it to:

- [ ] Fallback for platform with no emoji. Self-explanatory, just use the fallback property instead.
- [ ] Markdown. There's a few remark plugins for emojis, but none for custom ones. Easy enough to translate if we keep it as `:emoji:` (could start there), harder if we want to provide a reference to the set that originated it. The [mdast specification](https://github.com/syntax-tree/mdast?tab=readme-ov-file#linkreference) could give a few ideas.
- [ ] Rich Text Facet. Bluemoji has[ its own RichText Facet](https://github.com/aendra-rininsland/bluemoji/blob/main/schema/blue.moji/richtext/facet.json). We've never made one of our own before, but why not try?