ui: Refactor main to be class-based, add getMain() global utility

When ESM (ECMAScript Modules) land, it will no longer be possible
to treat `imports.ui.main` as an object where properties (exports)
can be re-assigned. To handle this change, `imports.ui.main` is
refactored into a `Main` class which is the instantiated as a singleton
in  `main.js`. `main.js` uses a proxy to allow for circular references,
this emulates how ES module imports are "live" and can be updated.

ECMAScript Modules are entirely asynchronous so to ensure
`main`/`GnomeShell` can be conditionally accessed synchronously outside
of the imports system (e.g. in looking glass, extension utilities,
etc.), add global.getMain() to retrieve the "main" object in these
12 jobs for !2302 with ewlsh/refactor-main-for-esm in 4 minutes and 1 second (queued for 1 second)
latest merge request