Rollup merge of rust-lang#72294 - GuillaumeGomez:js-cleanup, r=kinnison

JS cleanup

The goal here is just to improve the source code a bit. I recommend to review one commit at a time, otherwise it might not make much sense. :)

The biggest commit is the second one: to prevent to have "global" variables declared in `main.js` (and thus prevent name conflict or overwriting), I moved such code into anonymous functions.

r? @kinnison

cc @rust-lang/rustdoc

Author: Dylan-DPC