Previously, embedders had to fetch the 'exec' property off the RegExp
prototype in order to call exec (and such calls involve two
transitions between C++ and JS).

This CL exposes a convenient RegExp::Exec method through the API.

Bug: v8:9695
Change-Id: I57a9174626143d26f2ea34676b8e55fef64932cf
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1864940
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64508}