MF2 support

[samhh]

https://github.com/tc39/proposal-intl-messageformat

A ways off, but something we should potentially support in the future. intlc will likely still provide a superior experience than plain JS/TS as with ICU especially for type safety.

This may mean adding a new key to each message specifying the format type. We may consider calling it "input" and renaming "backend" to "output". And/or it might mean supporting a new input format entirely alongside JSON.