improve the C API test

This includes margin and padding (for later) and testing the "public" API instead of the struct internals (since that's how they will be used by user agents.) There's also some slight reorganization such as moving Position struct into the properties (as well as creating a properties.h header that does something similar to kidn of mimic the Rust structure)

crates/gosub-bindings/src/wrapper/node.rs

@@ -1,5 +1,5 @@
-use gosub_engine::render_tree::{text::TextNode, Node};
 use gosub_engine::render_tree::properties::{Position, Rectangle};
+use gosub_engine::render_tree::{text::TextNode, Node};
 
 use crate::wrapper::{text::CTextNode, CNodeType};
 

src/render_tree.rs

@@ -5,8 +5,8 @@ use crate::html5::node::NodeData;
 use crate::html5::parser::document;
 use crate::html5::parser::document::{Document, DocumentHandle};
 
-use crate::render_tree::{properties::Rectangle, text::TextNode};
 use crate::render_tree::properties::Position;
+use crate::render_tree::{properties::Rectangle, text::TextNode};
 
 pub mod properties;
 pub mod text;

src/render_tree/properties.rs

@@ -88,4 +88,4 @@ impl Default for Position {
     fn default() -> Self {
         Self::new()
     }
-}
+}