feat: Added support for flatten

tests/flatten.rs

@@ -0,0 +1,24 @@
+use lua_typed::Typed;
+
+#[derive(Typed)]
+pub struct B {
+    pub hello: String,
+    pub world: bool,
+}
+
+#[derive(Typed)]
+pub struct A {
+    #[serde(flatten)]
+    pub b: B,
+    pub cool: u32,
+}
+
+#[test]
+fn flatten() {
+    insta::assert_snapshot!(<A as Typed>::generate_full().unwrap(), @r"
+    ---@class A
+    ---@field hello string
+    ---@field world boolean
+    ---@field cool integer
+    ");
+}