Implement more types & multiple parameters

[jwpjrdev]

Currently, the only valid type is strings. There's a whole bunch of others to implement (check out the docs).

In code, all builtin (soon to be function) arguments are just a single string. This makes for some very ugly code (there's no dedicated value struct or anything to hold data & manage types). After this is dealt with, the AST especially will hopefully be a lot more idiomatic.