Improve javy npm cli wrapper

[surma]

After trying our npm cli wrapper for a while, I have arrived at the conclusion that the model I chose originally is annoying for us and annoying for users. This PR updates the script accordingly. These are the highlights of the new behavior:

• The cli wrapper script's version is independent of the Javy CLI version
• The cli wrapper automatically grabs the latest release using the GitHub Release API
• If desired, you can force a particular release by setting FORCE_RELEASE
• If desired, you can build ToT by setting FROCE_FROM_SOURCE.
• The script has been refactored so that it can be shared with function-runner without major modifications. In a future step, I might turn this into a module that can be shared.