Also update instructions to match recent script changes.
Also add reproducible commit SHA to post download instructions to support publishing the Firefox DevTools extension.
* Migrate prepare-release-from-ci to new workflow
I added a `--releaseChannel (-r)` argument to script. You must choose
either "stable" or "experimental", because every build job now includes
both channels.
The prepare-release-from-npm script is unchanged since those releases
are downloaded from npm, nt CI.
(As a side note, I think we should start preparing semver releases using
the prepare-release-from-ci script, too, and get rid of
prepare-release-from-npm. I think that was a neat idea originally but
because we already run `npm pack` before storing the artifacts in CI,
there's really not much additional safety; the only safeguard it adds is
the requirement that a "next" release must have already been published.)
* Move validation to parse-params module
* Updated DevTools local development instructions to mention experimental build step
* Added a command to download latest experimental release (for DevTools)
* Updated build instructions for clarity
* Added build-for-devtools package alias
