The world editor is a set of tools that help on creating and modifying the game levels and related data and resources.
It is built using core functions written in C++, with most of the logic in Lua scripting language. Dear Imgui is used for the UI side of it.
If you need to use the old editor (deprecated in the near future) documentation here: Legacy World Editor.
The editor can be extended just like the game, new extensions can be added to it. See the programming documentation related to editor here: World Editor Coding
World Editor Screenshot