Add upgrade script to convert "Spline" node to "Path" -> "Spline from Points" (#2274)

* write document upgrade code to transfrom Spline node into Path -> Spline from Points

* fix only connecting to single output

* shift position of newly inserted Path -> Spline from Points node

* refactor

* remove all old Spline node code

* rename Spline from Points node to Spline

* Code cleanup

* Update the demo art to natively use the new Spline node

---------

Co-authored-by: Keavon Chambers <keavon@keavon.com>

node-graph/graph-craft/src/document.rs

@@ -576,6 +576,13 @@ impl DocumentNodeImplementation {
 		}
 	}
 
+	pub fn get_proto_node(&self) -> Option<&ProtoNodeIdentifier> {
+		match self {
+			DocumentNodeImplementation::ProtoNode(p) => Some(p),
+			_ => None,
+		}
+	}
+
 	pub const fn proto(name: &'static str) -> Self {
 		Self::ProtoNode(ProtoNodeIdentifier::new(name))
 	}