Add --no-prefetch cli flag

--no-prefetch will directly use `load(...)` instead of
just injecting the script sources which makes it easier to navigate
and investigate profiles.

- Support more complex command line args and flags
- Add --help to cli.js

Bug: 411303884
Change-Id: Ic4714f823a7ed1a6f1edcdbf2e00ce34786fbd9b
Reviewed-on: https://chromium-review.googlesource.com/c/external/github.com/WebKit/JetStream/+/6483190
Reviewed-by: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>