support regexp buffer boundaries

[sno2]

Hello, this implements the stage 3 RegExp Buffer Boundaries proposal: https://github.com/tc39/proposal-regexp-buffer-boundaries

I tested against this test262 PR: tc39/test262#4975

There were two failing tests from ./run-test262 -c test262.conf and ./run-test262-debug -c test262.conf:

test262/test/built-ins/RegExp/unicode_restricted_identity_escape_alpha.js:17: unexpected error: Test262Error: IdentityEscape in AtomEscape: 'A' Expected a SyntaxError to be thrown but no exception was thrown at all
test262/test/language/module-code/namespace/internals/super-access-to-tdz-binding.js:32: unexpected error: Test262Error: Expected a ReferenceError but got a TypeError
Result: 50/43781 errors, 3382 excluded, 6102 skipped, 2 new

The first failure is a test that had not been updated to allow these new escapes, so I have left a note on the test262 PR: tc39/test262#4975 (comment). The second seems to be a new test that is unrelated to my changes.

I have not updated the test262 commit in this repository because the test262 PR has not been merged yet.

Closes #544