Generally developers don’t use hex editors to write code. That’d be insane, like trying to run a program by poking your cpu with a needle. Instead, developers use a compiler (such as GCC for windows) to turn their human readable code into working machine code.
I don’t know how helpful this is well over a year later, but googling “game boy advance compiler” will probably get you places
edit: realized this was about the haxe programming language and NOT hex editors, but same idea. whoopsy!