Polyfill `confirm()` and possibly other similar APIs

[heyzec]

The frontend UI can call confirm sometimes, and when this happens within the extension WebView, it is ignored. Here's an example:

1. Open the Source Academy Panel and the VS Code Developer Tools pane
2. In the Webview, open any question in an assessment.
3. Click on the "Reset" button. The "Save" button should turn orange, indicating unsaved changes.
4. Navigate back by clicking on the Missions button (Fire icon).
   • In the standard frontend, the user is prompted on whether they want to discard the unsaved changed.
   • In the extension, nothing happens. The WebView does not navigate to the Missions page either. Instead, an error entry appears in the console:

     Ignored call to 'confirm()'. The document is sandboxed, and the 'allow-modals' keyword is not set.
     

We need to polyfill this confirm function so that it appears as a VS Code notification. Note that confirm is blocking, though most VS Code APIs are non-blocking. Also, identify other such Web APIs that the frontend may call.

[heyzec]

Polyfilling looks like this: source-academy/frontend#3080 (comment)

Then, we will need to implement message passing from the frontend to the extension.