[regexp] Reorganize and deduplicate in the regexp parser

The parser is organized in a somewhat tricky way s.t. it can be
hard to map the implementation back to the specified grammar.

In particular, the logic for CharacterClassEscape, ClassEscape,
and CharacterEscape was implemented twice - once inside a character
class, once outside.

This CL refactors related logic to have only a single implementation.

As a drive-by, fix one related inconsistency related to \k inside
a character class.

Fixed: v8:10602
Change-Id: I5858840159694fa6f8d1aa857027db80754e3dfd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3178966Reviewed-by: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77114}

test/mjsunit/regress/regress-v8-10602.js

+// Copyright 2021 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+assertThrows(String.raw`/[\k](?<a>)/.exec()`);