This is because there are many filters using this feature. Moreover, it
could make it a little easier to add new arguments, just like dangling
commas in PHP arrays do.
Also re-align the CachingParser code of doLevelFunctions to the one in
the old Parser.
Bug: T153251
Change-Id: Ie4325159f47310788da57415a5e36e62aa4efad0
Some of them are actually too simple, and may be unuseful in tricky
situations. This patch adds a lot of test cases to provide an (almost)
bombproof safety with future patches.
Depends-On: I0bb1ed0109af66997e238b532d342d82d4c4ae19
Change-Id: I274ef306775c36be20acb662353f6537ff3f1a33