Make defined PHP constants to class-scoped constants

[technicalguru]

All constans currently set as define() shall be migrated to 'public const` class members (of a new const class). This will make the library namespaced completely.

[sabliao]

Hi @technicalguru , are you still maintaining this library? I was looking for a composer-installable version of the open phpqrcode library on sourceforge that I was testing with and liked, and yours seemed like the most direct one (i.e. wouldn't have to change too much on our end), but I also wanted to check if you've decided to abandon this or not (saw last commit was from last year

[technicalguru]

Hey @sabliao ! Yes I still do maintain it. If you have any improvements then higly welcome