Add linting for shell scripts
<!-- Please describe your changes on the following line: -->
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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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.
