[8.x] Add Str::substrReplace() and Str::of($string)->substrReplace() methods for the PHP native substr_replace() function

[ralphjsmit]

This pull request adds the ability to use the substr_replace() on the Str and Stringable classes.

What & why

Developers can now use directly use Str::substrReplace(...) and Str::of($string)->substrReplace(...).

Substring replace can be used to insert a string at a certain position in another string (set the last parameter, the length, to 0).

$string = '1300';

$result = Str::substrReplace($string, ':', 2, 0); 
// '13:00'

$result = (string) Str::of($string)->substrReplace(':', 2, 0);
// '13:00'

It can also be used to replace the remainder of a string after a certain position:

$result = (string) Str::of('Laravel Framework')->substrReplace('– The PHP Framework for Web Artisans', 8);
// 'Laravel – The PHP Framework for Web Artisans'

The benefit for webdevelopers is that they now have access to the substr_replace() method through the Str facade. The biggest benefit is (in my opinion), is that it's also a perfect candidate to be part of the fluent Stringable methods.

I'm also willing to PR a docs update for this, but I don't know if maintainers usually take care of updating the docs or if that is a task for contributors.

This is my first PR to Laravel, so please let me know if I missed anything or if there's anything I can improve. Thanks!

[NewEXE]

It's NOT multibytes-ready.

$right = 'Ларавел – PHP-фреймворк';
$res = \Illuminate\Support\Str::substrReplace('Ларавел фреймворк', '– PHP-фреймворк', 8);
dump($right, $res);

// ^ "Ларавел – PHP-фреймворк"
// ^ "Лара– PHP-фреймворк"