Save file as JSON representation of root layer

Also added dependency on serde_json.
Snippet of what the file looks like

```json
{
  "nodes":{},
  "root": {
    "blend_mode":"Normal","cache":"","cache_dirty":true,
    "data":{"Folder":{"layer_ids":[0,1],"layers":[...], "visible":true
  },
  "version":0}
```

Author: azeemba

Cargo.lock

@@ -13,4 +13,5 @@ log = "0.4"
 
 kurbo = {version="0.8", features = ["serde"]}
 serde = { version = "1.0", features = ["derive"] }
+serde_json = { version = "1.0" }
 glam = { version = "0.16", features = ["serde"] }

core/document/Cargo.toml

@@ -65,8 +65,12 @@ impl Document {
 
 	// Returns string that represents the full document (not just svg)
 	pub fn serialize_document(&mut self) -> String {
-		// Manually constructing JSON. Should switch to serde_json
-		format!("{{\"version\": 0, \"svg\": \"{}\"}}", self.render_root())
+		let doc = serde_json::json!({
+			"version": 0,
+			"root": self.root,
+			"nodes": {}
+		});
+		doc.to_string()
 	}
 
 	/// Checks whether each layer under `path` intersects with the provided `quad` and adds all intersection layers as paths to `intersections`.