Not sure we need to do anything about this now, but it occurs to me that we don't have a good story for how an extension would offer improved support for these non-generic forms of XML.  If the generic "xml" language has already claimed that file extension, no one can add a new language for it.

So for example, if I wanted to make an extension that offers offers tag & attribute hinting for SVG I'd ideally be able to un-register ".svg" from the "xml" language and declare a new "svg" language to take over it.  (For code hinting specifically, you could do hacks to avoid defining a new language, but ultimately that approach will probably hit a wall).

This is relevant here since the more _specific_ XML formats we add to our generic XML language, the more likely this is to become an issue.  But as with my SVG example above, it's possible to hit this issue even on current master.