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/notebookTestMain.ts

@@ -4,7 +4,6 @@
  *--------------------------------------------------------------------------------------------*/
 
 import * as vscode from 'vscode';
-import * as path from 'path';
 import { smokeTestActivate } from './notebookSmokeTestMain';
 
 export function activate(context: vscode.ExtensionContext): any {
@@ -117,16 +116,4 @@ export function activate(context: vscode.ExtensionContext): any {
 		},
 		cancelCellExecution: async (_document: vscode.NotebookDocument, _cell: vscode.NotebookCell) => { }
 	}));
-
-	const preloadUri = vscode.Uri.file(path.resolve(__dirname, '../src/customRenderer.js'));
-	context.subscriptions.push(vscode.notebook.registerNotebookOutputRenderer('notebookCoreTestRenderer', {
-		mimeTypes: [
-			'text/custom'
-		]
-	}, {
-		preloads: [preloadUri],
-		render(_document: vscode.NotebookDocument, _request: vscode.NotebookRenderRequest): string {
-			return '<div>test</div>';
-		}
-	}));
 }