Add 'invariant' function for debugging

plugin/src/invariant.lua

@@ -0,0 +1,27 @@
+local Config = require(script.Parent.Config)
+
+local invariant
+
+if Config.isDevBuild then
+	function invariant(message, ...)
+		message = string.format(message, ...)
+
+		error("Invariant violation: " .. message, 2)
+	end
+else
+	function invariant(message, ...)
+		message = string.format(message, ...)
+
+		local fullMessage = string.format(
+			"Rojo detected an invariant violation within itself:\n" ..
+			"%s\n\n" ..
+			"This is a bug in Rojo. Please file an issue:\n" ..
+			"https://github.com/rojo-rbx/rojo/issues",
+			message
+		)
+
+		error(fullMessage, 2)
+	end
+end
+
+return invariant