Proper support for XHTML

[ExE-Boss]

XHTML uses the XML syntax, which has subtle incompatibilities with the HTML syntax.

The most notable being the ability to use the self‑closing tag syntax on all elements and <![CDATA[…]]> sections:
https://github.com/gsnedders/csswg-test/blob/09674c9c828263779fd60401587b129c792265cf/css-cascade-3/reference/ref-filled-green-100px-square.xht

The current html.tmbundle is configured to be used for XHTML files, breaking the syntax highlighting of XHTML files using XML‑only syntax:

html.tmbundle/Syntaxes/HTML.plist

Lines 5 to 14 in 6a6fb29

	<key>fileTypes</key>
	<array>
	<string>html</string>
	<string>htm</string>
	<string>shtml</string>
	<string>xhtml</string>
	<string>inc</string>
	<string>tmpl</string>
	<string>tpl</string>
	</array>