servo: Merge #12295 - Add linting for shell scripts (from jimberlage:12158/shell-linting); r=aneeshusa

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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.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [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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Source-Repo: https://github.com/servo/servo
Source-Revision: 4bc629b369242d16675161ab6af8141dab0bb556

servo/python/tidy/servo_tidy/licenseck.py

@@ -23,6 +23,14 @@ licenses = [
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 """,
 
+"""\
+#!/usr/bin/env bash
+
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+""",
+
 """\
 #!/usr/bin/env python