Switch webview service-worker to use message channel (#138811)

* Switch webview service-worker to use message channel

This change hooks the service worker used for loading webview resources directly up to the main VS Code process over a message channel. Previously this communication had to go through an extra hop through the webview

This simplifies the logic somewhat (although this change required adding extra logic to exchange the message port). It also improves performance a little

* Update webview content commit version