Small DX improvements that goes with #60105 and #60083.

This PR adds two new methods, first() and last(), added to JsonPath builder. This voluntary reminds methods from the DomCrawler component. The goal is not to add every possible method, but I think first() and last() are common enough to be added.

I also propose to rename anyIndex() to all().

$path = new JsonPath();

// Get the first user of the collection
$path = $path->key('users')->first();

$path = new JsonPath();

// Get the last user of the collection
$path = $path->key('users')->last();

$path = new JsonPath();

// Get all users of the collection
$path = $path->key('users')->all();