Auto merge of #14166 - Wafflespeanut:tidy, r=frewsxcv

Allow tidy to run custom project-specific lints

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Since tidy is a package, it shouldn't contain our WPT lint. This PR changes the file list generator (we already have) into an object, and allows tidy to run custom python lint scripts (specified in the config file) under the context of `LintRunner`. The errors are then chained into our mechanism.

r? @frewsxcv (cc @jdm @edunham @aneeshusa and anyone else interested in reviewing it)

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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors

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- [x] There are tests for these changes

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