The IDE in question is for programmers who develop programs.
What you are looking for to create your own game system is to use the HL Authoring Kit. The Authoring Kit is what offers you the ability to create your own game system from scratch. In order to do so, you'd need to use a text editor like:
- Notepad
- Notepad++
- And other known options
You are able to access the .dat, .str and other file extensions within your newly created game system using just the text editor. However, as you keep track, having a Word Processor document to keep track of all your changes is helpful.
An IDE isn't the best way to keep track of building a game system within a program that's already programmed. It's more for software development.