Reapply "Removed spans feature from default"

This reverts commit 5121318.

Author: Ertanic

fluent-syntax/Cargo.toml

@@ -37,7 +37,7 @@ serde_json.workspace = true
 glob = "0.3"
 
 [features]
-default = ["spans"]
+default = []
 spans = []
 serde = ["dep:serde"]
 json = ["serde", "dep:serde_json"]