Add tabIndex to xterm-accessibility

See microsoft/vscode#135920

src/browser/AccessibilityManager.ts

@@ -55,6 +55,7 @@ export class AccessibilityManager extends Disposable {
     this._accessibilityTreeRoot = document.createElement('div');
     this._accessibilityTreeRoot.setAttribute('role', 'document');
     this._accessibilityTreeRoot.classList.add('xterm-accessibility');
+    this._accessibilityTreeRoot.tabIndex = 0;
 
     this._rowContainer = document.createElement('div');
     this._rowContainer.setAttribute('role', 'list');