feature: webview without service workers

Currently, in VSCode, webviews are implemented via two iframes and a service worker.

It seems it could be possible to improve webview loading performance and fix errors by using a single iframe, instead a service workers and two iframes.

Note: The change would only affect VS Code electron, not VS Code in the browser.

### Benchmarks

Benchmarks from a prototype PR using a single iframe indicate ~25% percent load time improvement for webviews. For example for the Codex webview:

### Other Benefits

Another benefit would be that it avoids a lot of weird webview service workers errors like:

- https://github.com/microsoft/vscode/issues/326092
- https://github.com/microsoft/vscode/issues/125993

### Breaking changes

It seems there shouldn't be any breaking changes. The feature would be opt in for extensions.