Add hasSelection public API

src/SelectionManager.ts

@@ -145,6 +145,13 @@ export class SelectionManager extends EventEmitter {
     this._buffer = buffer;
   }
 
+  /**
+   * Gets whether there is an active text selection.
+   */
+  public get hasSelection(): boolean {
+    return !!this._model.finalSelectionStart && !!this._model.finalSelectionEnd;
+  }
+
   /**
    * Gets the text currently selected.
    */