Bump js-yaml from 3.9.1 to 3.13.1 in /generator-exercise

Bumps [js-yaml](https://github.com/nodeca/js-yaml) from 3.9.1 to 3.13.1.
- [Release notes](https://github.com/nodeca/js-yaml/releases)
- [Changelog](https://github.com/nodeca/js-yaml/blob/master/CHANGELOG.md)
- [Commits](https://github.com/nodeca/js-yaml/compare/3.9.1...3.13.1)

Signed-off-by: dependabot[bot] <support@github.com>

generator-exercise/package-lock.json

@@ -2914,9 +2914,9 @@
       "dev": true
     },
     "js-yaml": {
-      "version": "3.9.1",
-      "resolved": "https://registry.npmjs.org/js-yaml/-/js-yaml-3.9.1.tgz",
-      "integrity": "sha512-CbcG379L1e+mWBnLvHWWeLs8GyV/EMw862uLI3c+GxVyDHWZcjZinwuBd3iW2pgxgIlksW/1vNJa4to+RvDOww==",
+      "version": "3.13.1",
+      "resolved": "https://registry.npmjs.org/js-yaml/-/js-yaml-3.13.1.tgz",
+      "integrity": "sha512-YfbcO7jXDdyj0DGxYVSlSeQNHbD7XPWvrVWeVUujrQEoZzWJIRrCPoyk6kL6IAjAG2IolMK4T0hNUe0HOUs5Jw==",
       "dev": true,
       "requires": {
         "argparse": "^1.0.7",