[9.x] Adds fragmentIf() method

[arkoe]

Adds a fragmentIf() method to conditionally return a fragment of a view. This solves the issue of normal request versus xhr requests sent by a library such as htmx.

Before:

if (request()->hasHeader('HX-Request')) {
    return view('products.index')->fragment('products-list');
}

return view('products.index');

After:

return view('products.index')->fragmentIf(request()->hasHeader('HX-Request'), 'products-list');