build: Introduce stylelint

build: Introduce stylelint

Initially, with just a single rule to make this a smaller change that we can
incrementally extend up to the full Wikimedia coding conventions preset; namely,
forcing hex references to colours to be in lower-case.

Note the odd src definition to avoid referring to mediawiki.less the directory in
the glob.

Change-Id: I34f0c8e3c8280e3fce0aa22d7537100b850cbb8d