(bug 44459) Implement mw.message.text():

* Change default message format in mediawiki.js from plain to text (this includes behavior of mw.msg shorthand).
* Code changes are primarily in jqueryMsg, with minor supporting changes in mediawiki.
* Organize tests by public entrypoint (mediawiki or mediawiki.jqueryMsg).
* Additional tests and assertions for all public entry points, for new and existing behavior.
* Convenience test assertion methods
* Add explanatory comments for new and existing code.
* Add setting to jqueryMsg for format, defaulting to parse, to preserve existing behavior for direct callers to jqueryMsg's public API.
* Since there are now two ways of constructing the abstract syntax tree, add a new parameter to the cache's key scheme.
* Minor formatting

Change-Id: I0d220692262356a12e2f1c0ce30cf6f090428332