Auto merge of #12295 - jimberlage:12158/shell-linting, r=aneeshusa

Add linting for shell scripts

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This changes tidy to check shell scripts for the proper shebang and
options.  It does not check that variables are formatted correctly.  It
also adds a check for the MPL 2.0 license in shell scripts.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #12158 (github issue number if applicable).

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- [X] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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