Add support for postfix operators (required for the factorial operator)

[alecrios]

@jozsefsallai Turns out I missed a big issue with the factorial operator, so I reverted that bit while keeping the factorial() method.

The issue is that ! is a postfix operator. At the moment, we only support prefix and infix operators, so we would need to make special modifications to the convert() function in src/index.ts.

Your 2 + 4! test did pass, although, try running 4! + 2 and see how things break down.

If you're willing to look into how to add postfix support to the convert() function, that would be appreciated. I'll be looking into it as well, though maybe not immediately.

[jozsefsallai]

Yeah, I figured there would be issues with that at some point. I saw that it worked with infix, so I went with that because I couldn't find anything for postfix operators. Can't believe I missed trying stuff like 4! + 2 🤦‍♂ Will try to look into this in my free time! Thanks for the merge!