Add document role to accessibility tree root

See microsoft/vscode#98918

src/browser/AccessibilityManager.ts

@@ -53,6 +53,7 @@ export class AccessibilityManager extends Disposable {
   ) {
     super();
     this._accessibilityTreeRoot = document.createElement('div');
+    this._accessibilityTreeRoot.setAttribute('role', 'document');
     this._accessibilityTreeRoot.classList.add('xterm-accessibility');
 
     this._rowContainer = document.createElement('div');