Microsoft provide a free IDE called Visual Studio Community : https://visualstudio.microsoft.com/vs/express/ Although intended for full application development, it offers some features that make it useful for developing scripts in the game. If thats all you want it for, don't bother adding any of the extra 'workload' features and then its not too big of a download.

Once installed, create a 'New project' from the File Menu, and select an 'Empty project' from the templates. (Depending on your version, this may be in the Visual C++ category).

Right click the project in the solution explorer window and choose Add → Existing item then browse to your script file(s) for your world. You can add multiple script files that are #included in your ServerScript.mit file, and using the solution explorer makes it easier to organise the files relevant to your world.

The IDE can also provide some useful text highlighting : Go to the Tools → Options menu, open up “Text Editor” in the left-side panel and select “File Extensions”. In the right-hand panel, type “mit” into the Extensions then click Add.

Though the script language doesn't do everything quite as Visual studio expects, it will detect things like comments and if statements and will highlight them in different colours, which makes your scripts much more readable.