Add support for loading custom extensions in compiled projects

[MikeDev101]

Projects that use experimental extensions will throw an error stating SecurityError: Failed to construct 'Worker': Script at (file:///) cannot be accessed from origin 'null'. Since the packager is a modified version of TurboWarp, how hard would it be to implement experimental extension support?

[GarboMuffin]

Using the new custom JS feature, you can do, eg.

vm.extensionManager.loadExtensionURL("https://extensions.turbowarp.org/fetch.js")

and it will load your extension

(there were also other fixes to make custom extensions actually work in the packager)