tie lint output into github annotations

This issue has been migrated from [#10622](https://github.com/matrix-org/synapse/issues/10622).

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In github actions, it's possible to define some regexes which get matched against the script output, and allow individual lines in the PR to be annotated accordingly. It would be nice to do this for our linters.

See https://github.com/actions/toolkit/blob/master/docs/problem-matchers.md, and https://github.com/TrueBrain/actions-flake8/blob/main/action/flake8-matcher.json for an example

Start with the GitHub Actions problem matchers documentation and the linked TrueBrain flake8 matcher example, then inspect Synapse's lint workflows to identify where linter output is emitted. Done means lint failures produce matching inline annotations on pull-request lines; verify the workflows with a representative lint failure.

Written by the indexing model from the issue text.