Add MB-339A unit yaml.

Not adding the MB-339APAN because that's an aerobatic model, which isn't
particularly useful in Liberation, but is potentially confusing to
people that don't know that.

https://github.com/dcs-liberation/dcs_liberation/issues/2511

resources/units/aircraft/MB-339A.yaml

@@ -0,0 +1,8 @@
+introduced: 1979
+manufacturer: Aermacchi
+origin: Italy
+price: 10
+role: Light Attack
+max_range: 200
+variants:
+  MB-339A: {}