[8.x] Adds Str::remove and Str::of('foobar')->remove methods

[lukeraymonddowning]

It always annoyed me that there was no str_remove method in PHP. I know that you can do str_replace and pass '', but I think it makes the code a fair bit harder to parse visually.

This PR adds remove($value) to the Str and Stringable classes.

Usage

// Fbar
Str::remove('o', 'Foobar');
Str::of('Foobar')->remove('o');

// Fbr
Str::remove(['o', 'a'], 'Foobar');
Str::of('Foobar')->remove(['o', 'a']);

I think it makes it much easier when dealing with complex string manipulation to quickly get a sense of what's happening.

This PR also adds an extra layer of functionality by allowing the removal of items in a case insensitive manner by passing false as the last parameter:

// oobar
Str::remove('f', 'Foobar', false);
Str::of('Foobar')->remove('f', false);