[8.x] Add chunk() to Fluent Strings

[PHLAK]

Adds the chunk() method to fluent strings allowing a string to be "chunked" by a specified length and returning a collection of those chunks.

Example Usage

Str::of('foobarbaz')->chunk(3); // Returns collect(['foo', 'bar', 'baz'])

This is particularly useful when chaining additional methods.

Str::of('FooBarBaz')->lower()->chunk(3)->implode('-'); // Returns 'foo-bar-baz'

I decided to just open a PR instead of starting a discussion first because it was such a minor (non-breaking) change. Totally understand if this is not wanted.

[bonzai]

I think it would be better to use mb_str_split, because str_split can't handle multibyte strings correctly:

   str_split('abc', 3)  // ["abc"]
mb_str_split('abc', 3)  // ["abc"]

   str_split('고마워', 3) // ["고", "마", "워"]
mb_str_split('고마워', 3) // ["고마워"]

   str_split('고마워', 2) // [b"ê³", b" ë", b"§ˆ", b"ì›", b"Œ"]
mb_str_split('고마워', 2) // ["고마", "워"]

[PHLAK]

@bonzai Sure, I'll update that.

EDIT: Done