This is a refactor-only change in preparation for the upcoming
builtins table split.

- Define fields through a macro list to avoid some manual boilerplate
  code.
- Consistent names for builtin_entry_table_ and builtin_table_, and
  update names of related methods as well.
- Add Builtins::ToInt to replace manual static_casts.
- Move around IsolateData methods s.t. they're in the same order as
  the underlying fields.

Bug: v8:12203
Change-Id: I68cd036b8de1dd2708e2d4579d76bb3baaea5e1c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3162128Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76874}