Add parser method to call parser functions

There is currently no straightforward way for anything to call a parser
function and get the result. This abstracts out that portion of
braceSubstitution() to allow this.

The immediate motivation for this patch is to close bug 41769 against
Scribunto, see I0138836654b0e34c5c23daaedcdf5d4f9d1c7ab2.

Bug: 41769
Change-Id: I339b882010dedd714e7965e25ad650ed8b8cd48f