💢 Tighter integration with GitHub Actions

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Relevant documentation: https://docs.github.com/en/actions/using-workflows/workflow-commands-for-github-actions

Errors, warnings, and notices

We can output messages in a format that GitHub Actions understands. For example, output

::warning file=test/a.cooltt,line=3,endLine=4,title=taste it::your Ch'i is critically low

to mark lines 3-4 of test/a.cooltt directly on GitHub.

Grouping

Not sure how useful it is, but we can use ::group:: and ::endgroup:: to have folders.

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Handled by RedPRL/asai#2.