This commit adds the 'l' (linear) RegExp flag (as in e.g. /asdf|123/l)
that forces execution in linear time.  These regexps are handled by the
experimental engine.  If the experimental engine cannot handle the
pattern, an exception is thrown on creation of the regexp.

The commit also adds a new global V8 flag and changes an existing one:
* --enable-experimental-engine, which turns on recognition of the RegExp
  'l' flag.  Previously this flag also caused all supported regexps to
  be executed by the experimental engine; this is not the case anymore.
* --default-to-experimental-regexp-engine takes over the previous
  semantics of --enable-experimental-regexp-engine:  We execute all
  supported regexps with the experimental engine.

Cq-Include-Trybots: luci.v8.try:v8_linux64_fyi_rel_ng
Bug: v8:10765
Change-Id: I5622a89b19404105e8be280d454e9fdd63c003b3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2461244Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Reviewed-by: Simon Zünd <szuend@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Martin Bidlingmaier <mbid@google.com>
Cr-Commit-Position: refs/heads/master@{#70892}