Add support for `return` statements

[FilipToth]

What this does

We finally allow for return statements in our functions!

TODO:

• [ ] Do control flow analysis

Syntax

function add(x : int, y : int) : int {
    return x + y
}

function hello(name : string) {
    if (name == "test")
        return

    var message = "Hi, " + name + "!"
    print(message)
}