Create adapters for Emoji

[essential-randomness]

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?