[12.x] Add missing separators to Stringable::ucwords

[kichetof]

This PR add missing separators to Stringable::ucwords introduced in #57581 that allow custom separators when chaining.

Use case:

Str::of('LARAVEL-Framework')->lower()->ucwords('-'); // Laravel-Framework

I added related tests to StringableTest

[kichetof]

test failure seem to be not related with my PR

There was 1 failure:

1) Illuminate\Tests\Support\SupportStrTest::testWhetherTheNumberOfGeneratedCharactersIsEquallyDistributed
Failed asserting that 9496 matches expected 10000.

/home/runner/work/framework/framework/tests/Support/SupportStrTest.php:807

FAILURES!
Tests: 12348, Assertions: 37014, Failures: 1, Warnings: 1, Skipped: 481.
Error: Process completed with exit code 1.