JS tooling: run TS grunt tasks only when files changed

- using grunt-newer to prefix precommit tasks
- only got it to work for tslint and tsc

Not applied to:

- sasslint does not take the file arguments in a way that grunt-newer
  recognizes
- no-only-tests throws an error when used with `newer`, but it's
  sub-second runtime

scripts/grunt/default_task.js

@@ -17,8 +17,8 @@ module.exports = function (grunt) {
 
   grunt.registerTask('precommit', [
     'sasslint',
-    'exec:tslint',
-    'exec:tsc',
+    'newer:exec:tslint',
+    'newer:exec:tsc',
     'no-only-tests'
   ]);