Finalize notebooks API (part 1)

[jrieken]

This issue is tracking the finalization of the notebooks API. In the first round that should be

• the registerNotebookSerializer-function and its descendant types: NotebookSerializer, NotebookData etc
• the package.json contribution for serializers
• the createNotebookController-function and its descendant types: NotebookController, NotebookDocument etc
• the registerNotebookCellStatusBarItemProvider-function and its descendant types: NotebookCellStatusBarItemProvider

[jrieken]

Final remaining questions (most up-to-date list is todo@API in vscode.proposed.d.ts)

• Should we rename the namespace to notebooks.
• Should things like notebookDocuments and its events be in workspace, just like textDocuments?
• Should we rename NotebookDocument#viewType. It is a document type, similar to TextDocument#languageId and therefore notebookType might be better
• NotebookCellMetadata and NotebookMetadata - the latter is empty, the former only has collapse states for input/output. Should we extract what's known to use into a separate type and leave metadata all open and more raw, e.g go back to { [key:string]: any} type?
• NotebookOutput and NotebookOutputItem both have metadata and there is still an offline discussion if both are needed. There is tendency to only have it on the former
• DEFER Remove onDidChangeNotebookCellExecutionState API proposal  #124970 there is an event for NotebookCellExecutionState changes but there is no corresponding property on the cell type. That's a misalignment.
• DEFER Finalize notebooks API (part 1) #122922 should NotebookRendererScript already be finalized or should we wait for renderer IPC to go final, same for asWebviewUri, keeping proposed and merged into