URI of the document (as string for map keys).
Whether this document is from Datalayer (remote) or local file.
The webview that sent the message. Always available for both NotebookProvider and CustomEditorProvider.
OptionalwebviewThe webview panel (for CustomEditorProvider). Optional because NotebookProvider doesn't have panels.
Context object passed to all message handlers. Contains everything needed to handle a message from a webview.