Make use of JS Modules less confusing

It's not obvious how the code that includes JS
Modules in the frontend build works.

This adds lots of comments to explain the various
bits and flattens `modules/all.mjs` to just be a
single function that starts off the window.GOVUK
namespace.

Also removes `module/all.js` from the repo'. It's
an artefact used by the frontend build so
shouldn't be included as source code.

app/assets/javascripts/main.js

@@ -1,4 +1,4 @@
-window.GOVUKFrontend.initAll();
+window.GOVUK.Frontend.initAll();
 
 $(() => GOVUK.addCookieMessage());