extensibility: preload extension bundles in web app

Created by: tjkandala

Part of RFC 411: Sourcegraph extensions performance. Closes #23367 (closed).

Kick off parallel loading of extension bundles in the main thread before requests are made through importScripts() in the extension host Web Worker. Consequently, bundles are downloaded in parallel (importScripts downloads sequentially).

New waterfall

The biggest bottleneck remaining is the Extensions query, which we plan on persisting to localStorage, so soon we should be able to load extensions before even the file loads:

Old waterfall

Good riddance 😩️