Adds OPML import and export to the Yarns Microsub server.
Install, activate, and visit Tools > Yarns: Import OPML to import subscriptions from an OPML file. Or to download an OPML file with your current Yarns subscriptions.
Moreover, a public OPML endpoint will be created at wp-json/yarns-opml/v1/export.
I don't think Yarns supports storing feed names and formats (RSS, Atom, mf2, etc.), yet. Because of this, the URL of mf2 feeds, which are HTML, will end up in xmlUrl rather than htmlUrl. Simply can't tell them apart. (This doesn't have to be an issue, though.)
Yes, I'm using WordPress's JSON API to output XML. Thought it was just easier this way. (Alternatively, I could've defined a new rewrite rule, and map that to a query parameter, and look for that by means of a parse_request callback function. And flush rewrite rules when the plugin's first activated. But I chose not to.)