Separate handling of ${^PREMATCH} from $` etc

Separate handling of ${^PREMATCH} from $` etc

Currently the handling of getting the value, length etc of ${^PREMATCH}
etc is identical to that of $` etc.

Handle them separately, by adding RX_BUFF_IDX_CARET_PREMATCH etc
constants to the existing RX_BUFF_IDX_PREMATCH set.

This allows, when retrieving them, to always return undef if the current
match didn't use //p. Previously the result depended on stuff such
as whether the (non-//p) pattern included captures or not.

The documentation for ${^PREMATCH} etc states that it's only guaranteed to
return a defined value when the last pattern was //p.

As well as making things more consistent, this is a necessary
prerequisite for the following commit, which may not always copy the
whole string during a non-//p match.