Update specification text to align with recent spec changes in ECMA-262

[rbuckton]

There have been a number of changes to the specification text since this proposal achieved Stage 3, most notably the addition of the v flag, which changed a number of AOs in the RegExp pattern semantics sections. As a result of these changes, we may be able to drop the "Modifiers Record" in favor of the new "RegExp Record", which contains the same information.