.. and refactor js-regexp.h.

- Hide the generic DataAt/SetDataAt accessors and replace them by
  dedicated accessors. Use the common lower_case naming scheme for
  these.
- Shuffle around definitions in js-regexp.h s.t. they are in a
  meaningful order.
- Dedupe the source/flags accessors - these fields are stored both
  on the instance and on the data array. We keep only accessors for
  the instance. Previously, these were disambiguated through naming
  oddities (e.g. Pattern() returned data->source).

Change-Id: I3d53c8b095f0d59621ff779608438f7fa5e8c92a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3193534
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77138}

This CL adds support for regular expressions. To serialize a regular
expression, the pattern and flags are extracted and serialized as
strings.

Also, JSRegExp::StringFromFlags() is introduced to allow for
transforming back from JSRegExp::Flags to the corresponding flag string.
To verify that this implementation is on par with
RegExp.prototype.flags, unittests are introduced under
regexp/regexp-unittest.cc and RegExpBuiltinsAssembler::FlagsGetter()
is updated to include a slow path that calls JSRegExp::StringFromFlags()
through a runtime function.

Bug: v8:11525, v8:11706
Change-Id: I9cad4c464129ba1cbf64672130d8410730d7d679
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2878751
Commit-Queue: Vicky Kontoura <vkont@google.com>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74566}