Seconding Sublime Text. It's, well... sublime. It's technically paid, but it has the WinRAR infinite trial going for it at the moment. The most it'll ever do to stop you is ask you whether you want to buy it or not. If you can't stomach that, then Notepad++, as mentioned before, is a fan favorite among anyone looking for a free and effective text editor, and extending off Lord Olleus, I'm fairly certain it comes with built-in Lua support.
Not to be a pedant, but you did mention an IDE, which warrants some clarification. Generally, a text editor is not the same as an IDE. IDEs often include text editors, but they also are associated with built-in debuggers, build faculties, automation, etc. I doubt anything out there will be compatible with any Civ-related offshoot of Lua short of Firaxis's official modding tools, whenever they release them. I think I've heard that FireTuner from Civ5 is compatible with some of the Lua in Civ6, though, so if you're looking for quick Lua tests, that might hold you over until proper tools are released.