add: is_gliding (close #87)

from javadoc: Checks to see if an entity is gliding, such as using an Elytra.
https://hub.spigotmc.org/javadocs/spigot/org/bukkit/entity/LivingEntity.html#isGliding()

src/main/java/at/helpch/placeholderapi/expansion/player/PlayerExpansion.java

@@ -420,6 +420,8 @@ public boolean canRegister() {
                 return bool(player.isDead());
             case "is_flying":
                 return bool(player.isFlying());
+            case "is_gliding":
+                return bool(player.isGliding());
             case "is_inside_vehicle":
                 return bool(player.isInsideVehicle());
             case "is_leashed":