Add whereModal and whereBaseRouteUrl helpers

[TylerFiekens]

Hello!

I've added a few macros that help with testing on the \Inertia\Testing\AssertableInertia class.

Here's some examples of how the can be used.

Before:

it('uses the correct modal', function () {
    $user = User::factory()->create();

    $this->actingAs($user)
        ->get(route('customers.create'))
        ->assertInertia(fn (AssertableInertia $page) => $page
            ->where('_inertiaui_modal.component', 'Customers/Create')
        );
});

it('uses the correct base route', function () {
    $user = User::factory()->create();

    $this->actingAs($user)
        ->get(route('customers.create'))
        ->assertInertia(fn (AssertableInertia $page) => $page
            ->where('_inertiaui_modal.baseUrl', route('customers.index'))
        );
});

After:

it('uses the correct modal', function () {
    $user = User::factory()->create();

    $this->actingAs($user)
        ->get(route('customers.create'))
        ->assertInertia(fn (AssertableInertia $page) => $page->whereModal('Customers/Create'));
});

it('uses the correct base route', function () {
    $user = User::factory()->create();

    $this->actingAs($user)
        ->get(route('customers.create'))
        ->assertInertia(fn (AssertableInertia $page) => $page
            ->whereBaseRouteUrl(route('customers.index'))
        );
});

[SuperDJ]

Perhaps adding some tests to confirm functionality will help get this merged