Add Rust-esque macros

[MysteryBlokHed]

The text replacement definitions can be useful, but adding Rust-like macros could also be useful. If you want to make a pack that lets users define their own functions to add functionality but you don't want to make them copy+paste a bunch of boilerplate, then you could use a macro to do it for them. Something like this could work:

!macro on_event
!takes $event
!takes $name
!takes $run
func $name
tag tick
obj $name $event
execute as @e[scores={$name=1..}] run $run
endfunc
!end

The above could be used as something like:

?on_event minecraft.used:minecraft.carrot_on_a_stick item_used say Hello!

The macro call would be converted to the following Databind code:

func item_used
tag tick
obj item_used minecraft.used:minecraft.carrot_on_a_stick
execute as @e[scores={item_used=1..}] run say Hello!
endfunc

The syntax is just an idea, so there might be better ways to implement it. If this issue is closed, !def definitions should be removed.

[MysteryBlokHed]

I don't think the way I've gone about implementing this is this PR is the best way. I think I'll try to do something more typical to other languages, like this:

# a macro with no args
!def some_macro()
    say A macro with no args
!end

# a macro with one arg
!def another_macro($arg)
    say Hello, $arg!
!end

Support for string literal tokenization would also be needed, though I doubt I'll support escaping things like \n immediately. I'll probably support multiline strings by default, without really trying to.

Macro calls could look like this:

?some_macro()
?another_macro("World")

[MysteryBlokHed]

Another idea would be to add a specific kind of file that lets you define project-wide macros. That way people could make libraries for Databind that give you macros to help development. Something like a .macros or .m.databind file extension could work for this.