Typically you force a split. Configuration is one thing, plugins are another. Maybe you can reference plugins from your config, but not express logic there. Boom, now you have both.

It also means your plugin system doesn't have to choose an "easy" or "simple" language. You could use wasm if you wanted to and allow just about everything under the sun. Although, Lua is used for Roblox games so, maybe it's complex enough.