tildify.el: Better support for XML

tildify.el: Better support for XML

* lisp/textmodes/tildify.el  (tildify-string-alist)
(tildify-ignored-environments-alist): Add `nxml-mode' to the list of
supported modes since `xml-mode' is no longer a thing but just an
alias to the former.  Also include comments and insides of tags in
`tildify-ignored-environments-alist' for XML modes.  Finally, since
XML does not define “ ”[1], use a numeric reference for
a no-break space (namely “ ”)

[1] XML specification defines only a handful of predefined entities.
    The list is at <http://www.w3.org/TR/REC-xml/#sec-predefined-ent>
    and includes only &lt;, &gt;, &amp;, &apos; and &quot; (meaning <,
    >, &, ' and " respectively).  This is in contrast to HTML and even
    XHTML which defined a whole bunch of entities including “&nbsp;”.

* automated/tildify-tests.el (tildify-test--example-html): Add support
for generating XML code, so that…
(tildify-test-xml) …test can be added to check handling of XML
documents.