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/package.json

@@ -64,6 +64,7 @@
       {
         "viewType": "notebookCoreTestRenderer",
         "displayName": "Notebook Core Test Renderer",
+        "entrypoint": "./src/customRenderer.js",
         "mimeTypes": [
           "text/custom"
         ]