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.

jest.config.js

@@ -0,0 +1,10 @@
+export default {
+  setupFilesAfterEnv: ['jest-extended'],
+  testTimeout: 20000,
+  testMatch: [
+    '**/web_src/**/*.test.js',
+  ],
+  transform: {},
+  verbose: false,
+};
+