chore(build): fluent-syntax: Use explicit features.

With Rust 2024 edition coming, we should prefer explicit features
over implicit features. After this change, `serde_json` no longer
shows up as a feature when doing a `cargo add` as the implicit
feature for it is no longer present.

fluent-syntax/Cargo.toml

@@ -38,7 +38,8 @@ glob = "0.3"
 
 [features]
 default = []
-json = ["serde", "serde_json"]
+serde = ["dep:serde"]
+json = ["serde", "dep:serde_json"]
 all-benchmarks = []
 
 [[bench]]