There isn't a current all-in-one editor for OpenXcom rulesets. Like Feru said, .rul files work in extended notepad type programs meant for coding.
There's one in this thread with a bit of extra help; a plugin for this port's code specifically. I'd recommend using it and the existing code in openxcom's standard\xcom1 folder though.
I suggest starting this as a mod though, not a modification of the code in the folder I just mentioned. I can provide a base blank mod if you'd like, to simplify this.
Some of the extra port specific features are explained here:
https://www.ufopaedia.org/index.php/Ruleset_Reference_Nightly_(OpenXcom)
