Update bspack workflow for distributing BS6 compatible artifacts

[ryyppy]

We need a way to easily distribute bspacked reason_api.ml and reason_binary.ml files for BuckleScript v6 and above (4.06), mostly because we want to keep the Reason JS playground bundle up to date with most recent Reason releases.

This PR keeps the original workflow alive (with slight adaptions in the README), but marks it as a legacy build workflow (BuckleScript will not support 4.02 in the future). Right now the 4.02 bspack instructions fail, mostly due to breaking updates to Reason, but it's still good to keep everything around for now.

The new workflow is introduced as bspacks/reason_bspack406.sh script, which needs BSPACK_EXE to be defined (pointing to a locally built bspack for now). Example usage calls can be found in the script comments.

Also, the 406 script takes the version from the reason.json file (since it looks like the source of truth for Reason version numbers). Easier for the user to not make mistakes.

Any feedback would be valuable.

@thangngoc89

[ryyppy]

@jordwalke I think this is ready to be merged. I looked through @jaredly's old PR regarding his esy setup (#2153), but it seems that 1) there's still something broken with the menhir dep 2) it's a more complex change for me and I can't tell how much effort it will take to get this working without shaving a yak.

This solution is definitely working for BS contributors (the target group of this change anyways). Happy to improve things in the future as soon as we figured out what is needed?

\cc @chenglou just for good measure

[jordwalke]

Sounds good! If the bs-contributors are happy with the workflow, then it does the job well! Most important thing is that it's correct. Thanks for the contribution.

[jordwalke]

Thanks @ryyppy