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}