fix: use serde content for token enum

BREAKING CHANGE:
json output is no longer like:
```json
{"token": "Ingredient", "name": "foo", "amount": "1", "unit": "gr"}
```
now:
```json
{"token": "Ingredient", "content": {"name": "foo", "amount": "1", "unit": "gr"}}
```
The problem is that serde was failing to serialize enum variants that contained a single string

crates/recipe-parser/src/parser.rs

@@ -196,7 +196,7 @@ fn parse_backstory<'a>(input: &mut Input<'a>) -> PResult<&'a str> {
 #[cfg_attr(feature = "serde", derive(serde::Serialize))]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 // if you use `zod` for example, using a tag makes it easy to use an undiscriminated union
-#[cfg_attr(feature = "serde", serde(tag = "token"))]
+#[cfg_attr(feature = "serde", serde(tag = "token", content = "content"))]
 pub enum Token<'a> {
     Metadata {
         key: &'a str,
@@ -582,7 +582,19 @@ mod test {
         let serialized = serde_json::to_string(&token).expect("failed to serialize");
         assert_eq!(
             serialized,
-            r#"{"token":"Ingredient","name":"quinoa","quantity":"200","unit":"gr"}"#
+            r#"{"token":"Ingredient","content":{"name":"quinoa","quantity":"200","unit":"gr"}}"#
+        );
+    }
+
+    #[test]
+    #[cfg(feature = "serde")]
+    fn test_token_serialization_creates_right_payload_single_string() {
+        let token = Token::Word("holis");
+
+        let serialized = serde_json::to_string(&token).expect("failed to serialize");
+        assert_eq!(
+            serialized,
+            r#"{"token":"Word","content":"holis"}"#
         );
     }