fix: move utility types out of .d.ts file to fix exported declaration files (#6315)

src/tests/helpers/api.ts

@@ -19,6 +19,7 @@ import { newFreeDrawElement, newImageElement } from "../../element/newElement";
 import { Point } from "../../types";
 import { getSelectedElements } from "../../scene/selection";
 import { isLinearElementType } from "../../element/typeChecks";
+import { Mutable } from "../../utility-types";
 
 const readFile = util.promisify(fs.readFile);