Auto-build emhermesc.js with GitHub Action

[robhogan]

Motivation

• The current build instructions are out of date since Hermes required a two-stage build, and the suggested Docker base is deprecated.
• It's currently very difficult to replicate/verify this build without Buck
• It's a laborious process to update or review any updates to this file
• There's no good way to know what version of Hermes the current emhermesc.js was built from.
• We're overdue an update now, not least to fix the temporary patch in 4be4099

Change

See the diff on the Readme (TL;DR - this uses a GitHub action to perform the build based on hermes-github-ref, and auto-pushes to a PR / uploads the result / fails on a mismatch).

Test plan

I iterated this on my fork: https://github.com/rh389/metro

Job fails because the file is out of sync on main

https://github.com/rh389/metro/runs/5662424803?check_suite_focus=true

Job succeeds by pushing to a PR

robhogan#3

Artifacts are uploaded

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@rh389 has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[robhogan]

Closing this for now as there's some more work to do to find a nice workflow - it seems that we can't commit to forks from an action even when the PR author allows commits from maintainers.