Add compatibility layer from leafo/scssphp to scssphp/scssphp

Add compatibility layer from leafo/scssphp to scssphp/scssphp

The basic usage of this compatibility layer appears to work well:

A CSS file can be compiled from the default style, without needing any
additional adjustments.

What this compatibility layer cannot do is rewrite the Exceptions that
are thrown by scssphp/scssphp. A strict catch block catching a specific
Exception, such as `\Leafo\ScssPhp\Exception\ParserException` will not
match the aliased `ScssPhp\ScssPhp\Exception\ParserException`.

Resolves #3398