Add is_on_ground predicate

Author: Gegy

data/lt/predicate/is_on_ground.json

@@ -0,0 +1,15 @@
+{
+	"condition": "minecraft:inverted",
+	"term": {
+		"condition": "minecraft:entity_properties",
+		"entity": "this",
+		"predicate": {
+			"type": "minecraft:player",
+			"stepping_on": {
+				"block": {
+					"blocks": "air"
+				}
+			}
+		}
+	}
+}