#100308

Fix #102049

* notebooks: ensure preloads load before webview output is shown

This serializes code in the webview so that we make sure preloads load
before rending output. Since the rendering of HTML can be deferred,
we also potentially need to defer other actions like clearing and
focusing.

* fixup! rm debug code, comment

* notebooks: pipe renderer API postmessages to the renderer itself

Previously the postMessage on acquireNotebookRenderer API was just
a proxy to the global vscode postmessage. Now, it's linked to the
renderer and will cause an optional `onDidReceiveMessage` method on
the renderer to be called.

The message still _also_ goes to the global webview message handling
for advanced use cases, but this change allows the webview<->renderer
communication to be more nicely contained
and separate for most use cases.

* wip

* fixup! pr comments

Fix #99110