Positron Notebooks: Integrate with the extension API

[seeM]

When using Positron notebooks, parts of the extension API don't work e.g. vscode.window.activeNotebookEditor. This blocks some important features that we've implemented as builtin extensions like notebook debugging (#9251).

We should integrate Positron notebooks with the extension API.

[seeM]

Here's what I've found as the minimal contract to integrate with the extension API:

1. Register an INotebookEditor with the INotebookEditorService per IPositronNotebookInstance
2. EditorPane implements:
   1. getControl(): returning the above INotebookEditor
   2. getId(): returning an ID from a known list of notebook editors (we may have to update if statements in a few places to include our notebook editor ID, see getNotebookEditorFromEditorPane)
3. MainThreadNotebooksAndEditors tracks the state of the extension API via these parts of INotebookEditor:
   1. getId(): returning a UUID
   2. textModel.uri
   3. getSelections(): ICellRange[]
   4. visibleRanges: ICellRange[]
   5. getViewModel().viewType
4. MainThreadNotebookEditors also uses these parts of INotebookEditor:
   1. onDidChangeVisibleRanges
      1. updates vscode.NotebookEditor.visibleRanges
      2. fires vscode.window.onDidChangeNotebookEditorVisibleRanges
   2. onDidChangeSelection
      1. updates vscode.NotebookEditor.selection
      2. fires vscode.window.onDidChangeNotebookEditorSelection
   3. Stores the editor in a map, although that's never used for anything. Only disposed when this class is disposed
   4. For vscode.NotebookEditor.revealRange
      1. hasModel()
      2. getLength()
      3. cellAt()
      4. revealCellRangeInView()
      5. revealInCenter()
      6. revealInCenterIfOutsideViewport()
      7. revealInViewAtTop()
   5. For vscode.NotebookEditor.selections setter
      1. setSelections()
      2. setFocus()