[fuzz] Add new subcommand based interface to CLI [1/3]

This is a more Go idiomatic interface that also for the different
modes of operations to be accessed as subcommands.

For example, to call the generate mode:
`./tools/run fuzz generate ...`
instead of
`./tools/run fuzz -generate ...`

This regularizes the interface for all calls to be `./tools/run fuzz
<subcmd> ...` instead of the mix of flags that some take in an arg and
some don't, or in the case of 'run', are just assumed to be the
default.

Beyond the interface aesthetics this allows for mode specific help
text, and for significant internal refactorings/simplifications.

Because of the fact the CQ bots use this script, migrating from the
old to new cannot be a single patch, but needs to done in 3 steps:
introduce new interface, move users to new interface, and then remove
the old interface

Bug: 562061482
Change-Id: I62a76b29fd99eab6bb082c87e55f46641a9ee69c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/345555
Commit-Queue: Ryan Harrison <rharrison@chromium.org>
Commit-Queue: dan sinclair <dsinclair@chromium.org>
Reviewed-by: dan sinclair <dsinclair@chromium.org>
Auto-Submit: Ryan Harrison <rharrison@chromium.org>