Add lint to ensure substitutions use the full form

Check that any variable substitutions use the full ${VAR} form,
not just $VAR (but don't check for quoting yet).

python/tidy/servo_tidy_tests/shell_tidy.sh

@@ -7,3 +7,4 @@ set -o nounset
 # Talking about some `concept in backticks` # shouldn't trigger
 echo "hello world"
 some_var=`echo "command substitution"`
+another_var="$some_var"

python/tidy/servo_tidy_tests/test_tidy.py

@@ -53,6 +53,7 @@ class CheckTidiness(unittest.TestCase):
         self.assertEqual('script does not have shebang "#!/usr/bin/env bash"', errors.next()[2])
         self.assertEqual('script is missing options "set -o errexit", "set -o pipefail"', errors.next()[2])
         self.assertEqual('script should not use backticks for command substitution', errors.next()[2])
+        self.assertEqual('variable substitutions should use the full \"${VAR}\" form', errors.next()[2])
         self.assertNoMoreErrors(errors)
 
     def test_rust(self):