XHTML Extension Support for love perviews

[abose]

Discussed in #974

^{Originally posted by ScottDutton3 February 15, 2023}
Good to see Brackets still alive with Phoenix.

I use it primarily for ebook creation. I could use xhtml extension support. As it is, I have to change one file to html before it will preview, then turn it back to xhtml so I can preview all files with that extension. Eventually, Phoenix forgets that it CAN preview those files and I have to do the small hack again.

Thanks very much.