add eol to IMainCellDto, no more attach/dettach text document but use the actual (sync'd) document or a fallback cell document

* 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

notebooks - wire up onDidChange event from notebook providers, fyi @rebornix,  https://github.com/microsoft/vscode/issues/96403