[2.x] Allow deepMerge on custom properties

[mpociot]

This PR adds the ability to specify custom dot notated keys to use when deep merging data with Inertia.

For example, let's say you have a component that renders a user pagination via infinite loading using the WhenVisible component:

Route::get('/users', function () {
    return Inertia::render('Users/Index', [
        'results' => Inertia::deepMerge(User::paginate()),
    ]);
});

This works great while scrolling through the site as new users get appended, however when you perform any additional request that results in a redirect to the /users route, the current deepMerge strategy will append the users to the state.

This PR aims to fix this by allowing you to specify on which key(s) the data should be merged. In the case of a pagination, you would return:

Route::get('/users', function () {
    return Inertia::render('Users/Index', [
        'results' => Inertia::deepMerge(User::paginate(), 'data.id'), // <--
    ]);
});

This tells Inertia to try and merge the results.data array by looking at the id property of the underlying objects (if it exists).
If the property is the same, the target object will be replaced with the new one from the response - otherwise it will be appended.

The JS implementation can be found here: inertiajs/inertia#2344