Add preprocessor if statements

[MysteryBlokHed]

Adding something similar to C's #if and #endif directives would make it easier to enable or disable blocks of code. A boolean could be read from the vars.toml file, and then the code contained would only be compiled if the condition is true. Something like this:

!if &enable_cool_feature
    func a_function
        say Hello
    end
!else
    func a_function
        say Hi
    end
!end

With a vars.toml like this:

# Enables a cool feature in the datapack
enable_cool_feature = true

If you have a large project, this could make it easier for users to only enable the parts that they expect to use, meaning less functions come up in the in-game autocomplete and a smaller datapack size.