chore(fluent-syntax): Use explicit features (#365)

Author: waywardmonkeys

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]]