Add incrementExpensiveFunctionCount() methods

To allow Lua libraries to mark functions as expensive, add an
incrementExpensiveFunctionCount() method to Scribunto_LuaEngine that
will call the corresponding Parser method and throw an error if the
limit is exceeded.

Also allow libraries to do the same thing from Lua by calling
mw.incrementExpensiveFunctionCount().

Change-Id: I56fded32b1077eff3980371e9abc9b3b7581f7b5