json: fix json encode/decode with embed support (#22277)

vlib/json/json_encode_embed_test.v

@@ -0,0 +1,27 @@
+import json
+
+struct Json2 {
+	inner []f64
+}
+
+struct Json {
+	Json2
+	test f64
+}
+
+fn test_main() {
+	str := '{"inner":[1,2,3,4,5],"test":1.2}'
+	data := json.decode(Json, str) or {
+		eprintln('Failed to decode json, error: ${err}')
+		return
+	}
+	println(data)
+	assert data.inner.len == 5
+	assert data.inner[0] == 1.0
+	assert data.inner[4] == 5.0
+	assert data.test == 1.2
+
+	data_json := json.encode(data)
+	dump(data_json)
+	assert data_json == str
+}