[9.x] Make assertPath() accepts Closure

[villfa]

Hi,

I propose to modify AssertableJsonString::assertPath() in order to ease the way to write tests:

$this
    ->getJson('/api/whatever')
    ->assertOK()
    ->assertJsonPath('data.created_at', fn($date) => $date !== null)
    ->assertJsonPath('data.name', fn($name) => strlen($name) > 0)
;

At first I thought of adding a new method, but after struggling to find a right name, I realized that accepting closures in assertPath would not cause any BC break.

[BrandonSurowiec]

I think it is an awesome addition.