Utilized SortDirection enum from PHP 8.6

[okorneliuk]

Related to #579

Added a polyfill of upcoming PHP 8.6, used SortDirection enum from it instead of private constants in Column. Slightly extended public API so it is possible to use SortDirection directly in sortOn method.

[nyamsprod]

@okorneliuk thanks for the PR. While it would be nice to have this inside the package. The CSV package is meant to be used without dependencies so the best course of action would be to update this PR by including the polyfill directly inside the package.

[nyamsprod]

I would simply use PHP_VERSION_ID it is simple and fast

<?php

if (PHP_VERSION_ID < 80600) {
    enum SortDirection
    {
        case Ascending;
        case Descending;
    }
}

[okorneliuk]

If polyfill is already defined by another library in the project, we will probably get an exception of trying to redefine the enum. I prefer to keep this as it is.