notebooks: remove impure renderer API

This removes the initial notebook renderer API and keeps the 'pure'
renderer API described in #102644 and hacked-in previously.

Remaining work in this area, in no particular order:

- Add messaging context to postMessage as requested by Don (API proposal TBA)
- Cleanups around how state is managed internally in the backLayerWebView
- Deprecate the renderer `viewType` in favor of calling it the `id` or `rendererId`

Q: I kept around some of the "transform" functions since the mime type
picking happens there, not sure if there's a better place for this
to happen now, or whether these methods should simply be renamed.

extensions/vscode-notebook-tests/src/customRenderer.js

@@ -11,3 +11,11 @@ vscode.postMessage({
 		firstMessage: true
 	}
 });
+
+const notebook = acquireNotebookRendererApi('notebookCoreTestRenderer');
+
+notebook.onDidCreateOutput(({ element, mimeType }) => {
+	const div = document.createElement('div');
+	div.innerText = `Hello ${mimeType}!`;
+	element.appendChild(div);
+});