Add a new output format option to print errors as GitHub Actions Workflow commands

[edgarrmondragon]

Feature

Add a new --output=github option to format errors as Workflow commands, similar to --output=json added in https://github.com/python/mypy/pull/11396.

Pitch

For example, for this output in JSON format

$ mypy src --output=json
{"file": "src/mylib/client.py", "line": 759, "column": 20, "message": "Argument 1 to \"export_responses\" of \"Client\" has incompatible type \"str\"; expected \"int\"", "hint": null, "code": "arg-type", "severity": "error"}

we'd get the following output in workflow command format

$ mypy src --output=json
::error file=src/mylib/client.py,line=759,col=20::(`arg-type`) Argument 1 to "export_responses" of "Client" has incompatible type "str"

Related:

• https://github.com/python/mypy/issues/10816
• https://github.com/python/mypy/pull/11396
• https://github.com/python/mypy/pull/17611