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 could give a few ideas.
Rich Text Facet. Bluemoji has its own RichText Facet. We've never made one of our own before, but why not try?

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