(#320) - Parse XmlLang as an identifier, not a CSS string

CSS strings are quoted, like "foo".  Our type for the xml:lang
attribute was being parsed as a CSS string, so an attribute like

  xml:lang="es"

would be parsed incorrectly, as the inner string would have had to be

  xml:lang="'es'"

So, we parse this attribute as an identifier now.

Fixes GNOME/librsvg#320
3 jobs from xml-lang in 9 minutes and 5 seconds (queued for 120 minutes and 4 seconds)
Status Job ID Name Coverage
  Test
passed #119961
fedora test

00:07:51

 
  Lint
manual #119963
allowed to fail manual
clippy
passed #119962
rustfmt

00:01:14