Fix outlet matching for whitespace-separated data-controller values

[tchiadeu]

OutletSet#matchesElement tokenizes the data-controller attribute with a naive .split(" "), splitting only on single literal space characters:

private matchesElement(element: Element, selector: string, outletName: string): boolean {
  const controllerAttribute = element.getAttribute(this.scope.schema.controllerAttribute) || ""
  return element.matches(selector) && controllerAttribute.split(" ").includes(outletName)
}

When a data-controller value is written across multiple lines, for example, generated by a server-side template that formats the attribute with newlines and indentation for readability, every identifier after the first ends up with a trailing newline glued to it ("my-controller\n" instead of "my-controller"). .split(" ") never removes that newline, so .includes(outletName) returns false even though the controller is genuinely connected to that element. The result is a "Missing outlet element" error for an outlet that's actually present.

This is inconsistent with how Stimulus parses data-controller everywhere else. TokenListObserver (responsible for actually connecting controllers) already handles this correctly via .trim().split(/\s+/). This PR applies the same tokenization in OutletSet#matchesElement so outlet resolution matches controller-connection semantics.

Also adds a regression test reproducing a data-controller value split across multiple lines, verifying the outlet is still found.