Add hammer_model entity

fgd/point/hammer/hammer_model.fgd

@@ -0,0 +1,10 @@
+@PointClass appliesto(-engine) 
+	// TODO: add an icon sprite?
+	studioprop()
+= hammer_model: "Fake model entity that only appears in Hammer. Useful for scale references or the like."
+	[
+	model(studio) : "World Model" : : "The model to display."
+	skin(integer) : "Skin" : 0
+
+	@resources [] // Doesn't exist.
+	]

src/hammeraddons/bsp_transform/globals.py

@@ -123,6 +123,7 @@ def strip_ents(ctx: Context) -> None:
         'func_instance_parms',
         'func_instance_origin',
         'comp_player_input_helper',
+        'hammer_model',
     ]:
         for ent in ctx.vmf.by_class[clsname]:
             ent.remove()