Mark onDidChangeActiveTextEditor can fire with undefined

From #39413

**Bug**
onDidChangeActiveTextEditor may fire with undefined for the editor. This is not currently marked in the API

src/vs/vscode.d.ts

@@ -4496,7 +4496,7 @@ declare module 'vscode' {
 		 * has changed. *Note* that the event also fires when the active editor changes
 		 * to `undefined`.
 		 */
-		export const onDidChangeActiveTextEditor: Event<TextEditor>;
+		export const onDidChangeActiveTextEditor: Event<TextEditor | undefined>;
 
 		/**
 		 * An [event](#Event) which fires when the array of [visible editors](#window.visibleTextEditors)

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

@@ -20,13 +20,13 @@ export class ExtHostEditors implements ExtHostEditorsShape {
 	private readonly _onDidChangeTextEditorSelection = new Emitter<vscode.TextEditorSelectionChangeEvent>();
 	private readonly _onDidChangeTextEditorOptions = new Emitter<vscode.TextEditorOptionsChangeEvent>();
 	private readonly _onDidChangeTextEditorViewColumn = new Emitter<vscode.TextEditorViewColumnChangeEvent>();
-	private readonly _onDidChangeActiveTextEditor = new Emitter<vscode.TextEditor>();
+	private readonly _onDidChangeActiveTextEditor = new Emitter<vscode.TextEditor | undefined>();
 	private readonly _onDidChangeVisibleTextEditors = new Emitter<vscode.TextEditor[]>();
 
 	readonly onDidChangeTextEditorSelection: Event<vscode.TextEditorSelectionChangeEvent> = this._onDidChangeTextEditorSelection.event;
 	readonly onDidChangeTextEditorOptions: Event<vscode.TextEditorOptionsChangeEvent> = this._onDidChangeTextEditorOptions.event;
 	readonly onDidChangeTextEditorViewColumn: Event<vscode.TextEditorViewColumnChangeEvent> = this._onDidChangeTextEditorViewColumn.event;
-	readonly onDidChangeActiveTextEditor: Event<vscode.TextEditor> = this._onDidChangeActiveTextEditor.event;
+	readonly onDidChangeActiveTextEditor: Event<vscode.TextEditor | undefined> = this._onDidChangeActiveTextEditor.event;
 	readonly onDidChangeVisibleTextEditors: Event<vscode.TextEditor[]> = this._onDidChangeVisibleTextEditors.event;