Add warning about version compatibility when exporting schematic

Author: Bentroen

scripts/calculate_size/calculate_size.gml

@@ -1,5 +1,10 @@
 function calculate_size() {
 	var a, b, c;
+	if (warning_schematic == 0) {
+		message("The exported schematic is currently only compatible with Minecraft: Java Edition 1.12 and below. To import it in 1.13 and above, first create a world in 1.12, import the schematic, then open the world in 1.13 or above. Newer versions may be supported in the future.\n\nThis warning will never be shown again.", "Warning")
+		warning_schematic = 1
+		save_settings()
+	}
 	if (block_outside > 0) {
 	    if (!question("Some note blocks are outside of Minecraft's 2 octave limit. Do you want to export a Schematic anyway, ignoring the affected blocks?", "Minecraft Compatibility")) return 0
 	}