Change API RegExp::GetSource to return a string identical to ToString()
and RegExp.prototype.source.

Bug: v8:11693
Change-Id: I3d148883fe6f8a3ff49e552ddd72b1e92f52baf3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2900737Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Patrick Thier <pthier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74854}