Support for page load detection, title display, navigation commands, etc. in the Viewer on Posit Workbench

[jmcphers]

Currently, the Viewer pane uses Electron APIs to inject a bunch of window message handlers into every page that loads in the Viewer. You can find a writeup of how this works in this PR: #3430

You can find the full set of handlers we inject in this file: https://github.com/posit-dev/positron/blob/main/src/vs/workbench/contrib/webview/browser/pre/webview-events.js

Without these handlers, the Viewer pane functionality is significantly impaired. In particular:

• We don't know when the page is done loading.
• We can't read the title of the page (for displaying in the Viewer pane).
• We don't get custom behavior for clicking on links.
• Events are not forwarded outside the Viewer pane to Positron.
• Forward/back navigation may not work reliably.

For example, here's where we guess that the load time of the page is 500ms, because we don't have a way to know otherwise.

positron/src/vs/workbench/contrib/positronPreview/browser/previewOverlayWebview.ts

Lines 86 to 96 in de62b4e

	// Notify the parent window that the window has loaded. This
	// is a best-effort guess since we can't see cross-origin
	// into the frame.
	setTimeout(() => {
	window.parent.postMessage({
	channel: 'did-load-window',
	data: {
	title: ''
	}
	}, '*');
	}, 500);

Since we are injecting the handlers using Electron APIs, they are not injected in Posit Workbench. In order to achieve parity with the desktop experience, we need to get them injected somehow (or recreate the functionality in some other way). Some ideas:

• Ask Posit Workbench to inject the script, since Posit Workbench is already proxying the HTTP requests to the Viewer content.
• Take advantage of the fact that the cross-domain restriction probably doesn't apply on Workbench -- that is, the content in the Viewer probably has the same authority/security origin as the Positron UI, so try to use script to see directly into the Viewer frame.
• Create some sort of proxy layer of our own to inject the script.
• Run away from home and live in the woods.