refactor: use ES module imports in browser core (#4158)

With these changes a few browser-side core files now use native ES
modules. `Loader`, `MMSocket`, `Module` and `MM` can be imported
directly instead of being read off `window`. `main.js` and `loader.js`
are no longer wrapped in IIFEs - they're just normal modules now.

`Module`, `MM` and `MMSocket` are still exposed as globals, so
third-party modules that use the old API keep working.

The changes are mostly structural, behavior should stay the same. A few
internal helpers in `main.js` got an underscore prefix because their
names clashed with public `MM` methods.

## Why

The old setup relied a lot on script order: a file could use `Loader` or
`MMSocket` only because another script happened to put it on `window`
first. Imports make that explicit.

The bigger goal is to move away from the legacy script-loading patterns
- making it easier to understand and easier to test - in other words:
easier to maintain.

More of the core could be "cleaned up" the same way, but that would blow
up this PR.

For reviewing, I recommend to hide the whitespace changes.

index.html

@@ -51,11 +51,9 @@
     <script type="text/javascript" src="translations/translations.js"></script>
     <script type="text/javascript" src="js/translator.js"></script>
     <script type="text/javascript" src="config/basepath.js"></script>
-    <script type="text/javascript" src="js/module.js"></script>
-    <script type="text/javascript" src="js/loader.js"></script>
-    <script type="text/javascript" src="js/socketclient.js"></script>
     <script type="text/javascript" src="js/animateCSS.js"></script>
     <script type="text/javascript" src="js/positions.js"></script>
-    <script type="text/javascript" src="js/main.js"></script>
+    <script type="module" src="js/module.js"></script>
+    <script type="module" src="js/main.js"></script>
   </body>
 </html>