Fix unreachable sep rule in self-describing PEG grammar
The sep rule (for the % separated-list-zero-or-more operator) was defined in examples/peg.peg but never referenced in suffix, making it unreachable. Only %+ (one or more) was reachable via sep1. The suffix rule now includes sep.
Update to work with ponyc 0.61.1
ponyc 0.61.1 now detects exhaustive match blocks and treats unreachable else clauses as compile errors. peg has been updated to remove these unreachable else clauses, which means peg now requires ponyc 0.61.1 or newer.