cargo: use 'dep' for optional dependencies

This was an oversight. The 'js-sys' and 'wasm-bindgen' dependencies
only be referenced as dependencies. They aren't features themselves.
(And they were never documented as such, so this change is justifiable
in a semver compatible release.)

Cargo.toml

@@ -72,7 +72,7 @@ tzdb-zoneinfo = ["std"]
 # on Jiff directly and enable the `js` feature themselves.
 #
 # (This is the same dependency setup that the `getrandom` crate uses.)
-js = ["wasm-bindgen", "js-sys"]
+js = ["dep:wasm-bindgen", "dep:js-sys"]
 
 [dependencies]
 jiff-tzdb = { version = "0.1.0", path = "jiff-tzdb", optional = true }