plugin: webviews on development host do not work on electron

[saltylassi]

Bug Description:

On electron, and when starting the development host (plugin-system) webviews fail to properly start due to a service worker error:

Uncaught (in promise) DOMException: Failed to register a ServiceWorker: The document is in an invalid state.

Steps to Reproduce:

1. open https://github.com/microsoft/vscode-extension-samples/tree/main/webview-sample as a workspace
2. build the extension (npm i)
3. start the debug configuration run extension
4. the development host should start
5. trigger the command cat coding: start cat coding session
6. the webview editor opens but there is no content with the above error

Additional Info

• Operating System: Windows 10
• Theia Version: latest

[vince-fugnitto]

I simplified the issue so the problem is more easier to reproduce and identify :+1:

Looking at vscode, it seems they set special privileges for the vscode-webview scheme to allow service-workers, and it may be the reason that it does not work in the framework at the moment (previously I thought it was disabled for security reasons):

• https://github.com/microsoft/vscode/blob/d7bc87228955d47db801b71fff5eb86944b20475/src/main.js#L72-L75