new mutations + plant part data structure simplified

entities/plants/scripts/plant_mutation/rhizome_mutation.gd

@@ -0,0 +1,22 @@
+extends PlantMutation
+class_name RhizomeMutation
+
+func get_icon() -> Texture:
+    return preload("res://common/icons/alert-triangle.svg")
+
+func get_mutation_id() -> String:
+    return "RHIZOME"
+
+func get_mutation_name() -> String:
+    return tr("RHIZOME")
+
+func get_mutation_description() -> String:
+    return tr("RHIZOME_EFFECT_TEXT").format({
+        "digging_radius": get_digging_radius()
+    })
+
+func _start_maturation_effect(plant: Plant):
+    plant.region.dig_hole(plant.global_position, get_digging_radius())
+
+func get_digging_radius() -> int:
+    return level