Readonly properties for php 8.1

[xy2z]

Older php 8.0 users can still use the older version, so make a new php8.1 release with public readonly properties, so I don't need to make getters for every option.

Remember to update composer.json and README that it supports PHP 8.1 (but 8.0 can use version X)

Also consider making setters for each option, it can be nice to have to change an option after creation.