Add arc/rc features to koto_serialize

crates/serialize/Cargo.toml

@@ -9,7 +9,10 @@ homepage = "https://koto.dev"
 repository = "https://github.com/koto-lang/koto"
 keywords = ["scripting", "language", "koto"]
 
-# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+[features]
+default = ["arc"]
+arc = ["koto_runtime/arc"]
+rc = ["koto_runtime/rc"]
 
 [dependencies]
 serde = { workspace = true }