Context aware parsing of definition list colon

 * Ignore inside tags to prevent illegal overlapping.

 * Follows findColonNoLinks in the php parser.

 * Adds a new test to document this behaviour, which exposes a bug in
   parsing templates in definition lists. Serialization is introducing
   unnecessary nowikis, which looks like they should be fixed by
   https://gerrit.wikimedia.org/r/#/c/178871/ but probably aren't..

Bug: T71219
Change-Id: I2d8403eb732fdb022e8c87a990fd392a62e7f477