fix(common): locales/global/*.js are not ES5 compliant (#36342)

Although this code has been part of Angular 9.x I only noticed this
error when upgrading to Angular 9.1.x because historically the source
locale data was not injected when localizing, but as of
angular/angular-cli#16394 (9.1.0) it is now included. This tipped me off
that my other bundles were not being built properly, and this change
allows me to build a valid ES5 bundle (I have also added a verification
step to my build pipeline to alert me if this error appears again in any
of my bundles).

I found the `locales/global/*.js` file paths being referenced by the
`I18nOptions` in
@angular-devkit/build-angular/src/utils/i18n-options.ts,
and following that it looks like it is actually loaded and used in
@angular-devkit/build-angular/src/utils/process-bundle.ts. I saw the
function `terserMangle` does appear that it is likely aware of the build
being ES5, but I'm not sure why this is not producing a valid ES5
bundle.

This change updates `tools/gulp-tasks/cldr/extract.js` to produce ES5
compliant `locales/global/*.js` and that fixes my issue. However, I am
not sure if @angular-devkit/build-angular should be modified to produce
a valid ES5 bundle instead or if the files could be TypeScript rather
than JavaScript files.

A test that a valid ES5 bundle is produced would be helpful, and I hope
this is reproducible and not some issue with my config.

PR Close #36342

packages/common/locales/global/root.js

@@ -13,7 +13,7 @@
   global.ng = global.ng || {};
   global.ng.common = global.ng.common || {};
   global.ng.common.locales = global.ng.common.locales || {};
-  const u = undefined;
+  var u = undefined;
   function plural(n) { return 5; }
   global.ng.common.locales['root'] = [
     'root',