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

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.

---
<!-- 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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components/servo/fake-ld.sh

@@ -1,3 +1,15 @@
-#!/bin/bash
-TARGET_DIR=$OUT_DIR/../../..
-arm-linux-androideabi-gcc $@ $LDFLAGS -lc -o $TARGET_DIR/libservo.so -shared && touch $TARGET_DIR/servo
+#!/usr/bin/env bash
+
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+set -o errexit
+set -o nounset
+set -o pipefail
+
+TARGET_DIR="${OUT_DIR}/../../.."
+arm-linux-androideabi-gcc "$@" \
+                          "${LDFLAGS-}" -lc -shared \
+                          -o "${TARGET_DIR}/libservo.so"
+touch "${TARGET_DIR}/servo"