(feat): support # and @ as valid search predicates

[natask]

I would like to have search syntax that treats # and @ as tag predicate.
currently defined pexs expect : to follow predicates.

I would like to keep that while adding new type of predicates that apply as is.

Interface

Currently read from predicates variable and populate predicates within pex. This feature requires some predicates to be treated differently by peg. I could either add a flag to each special predicate, by use of a plist-key :bypass or :verbatim.