[6.x] Str::afterLast() and Str::beforeLast() helpers

[patrickomeara]

This PR adds afterLast() and beforeLast() to the Str helper.

These are useful in many situations, recently I have used it to find the type in database notifications, where morph maps cannot be used:

$type = 'App\Notifications\Tasks\TaskUpdated';
Str::afterLast($type, '\\'); // TaskUpdated

and to get the full file name (minus extension) when there are . in the filename. This is useful when processing many file paths from external services (s3) where loading each file is too slow.

$filename = 'photo.2019.11.04.jpg';
Str::beforeLast($filename, '.'); // photo.2019.11.04

Note: PR #28218 added these functions with a case insensitive flag.