.. 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}