Add hidden --parser option to override the input format, rather than inferring it from the file extension

[felix-cw]

Summary

This adds a hidden --parser option to the check command, motivated by #6847.
This option allows overriding the input format of the code, rather than inferring from the file extension.

The possible values are py and ipynb. If --parser is not specified, file formats are inferred from file extensions.

Test Plan

• Added tests in integration_tests.rs to ensure the parser override worked as expected.

[github-actions]

PR Check Results

Ecosystem

✅ ecosystem check detected no changes.

[charliermarsh]

This is really nice work. Let me try to get some alignment on what we want this argument to be called in the originating issue, since there was some discussion there still...

[charliermarsh]

(Assigning myself as reviewer.)

[felix-cw]

Thanks for the kind comment! I'm thinking that the preferred idea in the discussion is to provide an option that maps file extensions to language, whereas this just overrides the language for everything. Please don't hesitate to close this PR if this is the wrong approach.

[felix-cw]

I'm closing this PR in favour of #8373 which seems to be a more sensible approach.