If you've ever worked with a proper IDE before, it should come as no surprise to you that DM, the default "IDE" of BYOND is kind of awful. Seriously it breaks when you copy paste text with LF line endings into it.
While there's no proper replacement for mapping or managing icons (although I personally recommend you use an image editor like Paint.NET for spriting and you can use FastDMM for mapping), code editing is a ton easier. There's a variety of editors out there which are modular enough to be used for any language, the best one is Sublime Text 3.