Move webview to use onDidChangeViewState event

After discussions, we settled on making the webview private unlike `TextEditors`. This means that webview events will live on the webview object itself

Fixes #44571

src/vs/workbench/api/node/extHostTextEditor.ts

@@ -313,8 +313,6 @@ export class ExtHostTextEditorOptions implements vscode.TextEditorOptions {
 
 export class ExtHostTextEditor implements vscode.TextEditor {
 
-	public readonly editorType = 'texteditor';
-
 	private readonly _proxy: MainThreadTextEditorsShape;
 	private readonly _id: string;
 	private readonly _documentData: ExtHostDocumentData;