Get copy context menu state from correct place

This also relies on an sourcelair/xterm.js#724

Part of #29122

src/vs/workbench/parts/terminal/electron-browser/terminalPanel.ts

@@ -137,7 +137,8 @@ export class TerminalPanel extends Panel {
 				this._register(a);
 			});
 		}
-		this._copyContextMenuAction.enabled = document.activeElement.classList.contains('xterm') && window.getSelection().toString().length > 0;
+		const activeInstance = this._terminalService.getActiveInstance();
+		this._copyContextMenuAction.enabled = activeInstance && activeInstance.hasSelection();
 		return this._contextMenuActions;
 	}