Implement a parser for ircv3 and add unit tests to it.

This change got pretty big so I didn't implement unescapping tags yet. I did
however put the unit tests in for escaped tags, but they are currently #if 0'd
out.

The unit tests are based on the msg-split test cases from https://github.com/ircdocs/parser-tests/blob/master/tests/msg-split.yaml

Testing Done:
Ran the unit tests.

Bugs closed: PIDGIN-17585

Reviewed at https://reviews.imfreedom.org/r/1874/