Add frontend testing, require node 12 (#15315)

- Add basic frontend unit testing infrastructure using jest in ESM mode
- Rename 'make test' to 'make test-backend'
- Introduce 'make test-frontend' and 'make test' that runs both
- Bump Node.js requirement to v12. v10 will be EOL in less than a month.
- Convert all build-related JS files to ESM.

I opted to run frontend tests run as part of the compliance pipeline because
they complete fast and are not platform-specific like the golang tests.

package.json

@@ -1,8 +1,9 @@
 {
   "license": "MIT",
   "private": true,
+  "type": "module",
   "engines": {
-    "node": ">= 10.13.0"
+    "node": ">= 12.17.0"
   },
   "dependencies": {
     "@claviska/jquery-minicolors": "2.3.5",
@@ -50,6 +51,8 @@
     "eslint-plugin-import": "2.22.1",
     "eslint-plugin-unicorn": "29.0.0",
     "eslint-plugin-vue": "7.8.0",
+    "jest": "26.6.3",
+    "jest-extended": "0.11.5",
     "stylelint": "13.12.0",
     "stylelint-config-standard": "21.0.0",
     "svgo": "2.3.0",