make gmp optional

[tacman]

In #14, it seems that ext-gmp was removed, but it's still in composer.json.

Any chance this could be optional? I'm running PHP 8.3 on the latest Ubuntu, and having a hard time installing it. I've tried

  sudo apt install php-gmp

But I get

sudo apt install php-gmp
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 php8.2-gmp : Depends: php8.2-common (= 8.2.10-2ubuntu1) but 8.2.15-1+ubuntu22.04.1+deb.sury.org+1 is to be installed
E: Unable to correct problems, you have held broken packages.

[ip2location]

The GMP has been removed from the required package.