Adds a custom class for matched brackets to allow better integration
with custom bracket styles. The brackets won't be bold in the 2017WTE.
Bold font might lead to misalignment there. See ticket.
Note: box-shadow seems to be supported for quite some time by all
relevant browsers
Bug: T270926
Change-Id: Ica1e301f63a106a96db3bfaba4b2f322af64b009
Basic tests to show that the highlighting classes have been attached
to the expected elements.
TODO in later patches:
* tests for the wikitext 2017 editor
Bug: T270240
Change-Id: I01ebd9881d38dd877f19ee3bb4fdcbb74d43afaf
