Add clearSelection API

src/SelectionManager.ts

@@ -180,6 +180,16 @@ export class SelectionManager extends EventEmitter {
     return result.join('\n');
   }
 
+  /**
+   * Clears the current terminal selection.
+   */
+  public clearSelection(): void {
+    this._model.selectionStart = null;
+    this._model.selectionEnd = null;
+    this._removeMouseDownListeners();
+    this.refresh();
+  }
+
   /**
    * Translates a buffer line to a string, with optional start and end columns.
    * Wide characters will count as two columns in the resulting string. This