"Temporarily" convert ?-quantified captures to use **0..1 instead.

This is in preparation for bringing the ?-quantifier in line with
S05, such that ? is an "item quantifier" instead of a "list quantifier".
Start adding tests for quantified captures.
Fix up incomplete quantified captures commit 54a9a07.
Fix a version check thinko.
Fix bigint_is_prime via intermediate int result (RT #117731)
P6regex: Switch ?-quantifier to be an item quantifier instead of a list quantifier.

In other words, using ? on a capturing expression no longer produces
a list of captures; it's a single capture (or Nil/nonexistent).
Update parrot bootstrap with ?-quantifier changes.
Convert NQPRegex.ACCEPTS to be multimethods based on concrete-ness.
let github render the readme prettily
Merge branch 'master' into rak-jvm-support
Update bootstrap after merging from master.