Add Case-sensitive flag for Stringable replace

[rudashi]

Examples:

// Case-insensitive Stringable example
// foo/foo/foo
Str::of('x/x/x')->replace('X', 'foo', false);
 
// Case-insensitive
// foo bar laravel
Str::replace('baz', 'laravel', 'foo bar Baz', false);
 
// Default case-sensitive will not replace `Baz` with `laravel`
Str::replace('baz', 'laravel', 'foo bar Baz'); // foo ba