Mark js-sys as optional for identity_core

In iotaledger#1397 my intention was to make the `js-sys` dependency mutually
exclusive with the feature `custom_time`. As it turns out, it's not that
simple and mixing target specific dependencies with feature specific
dependencies does not actually work. See [here](https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#platform-specific-dependencies)
and [here](https://doc.rust-lang.org/cargo/reference/features.html#mutually-exclusive-features).

So this removes the broken feature reference in the dependency declaration
and instead marks it as `optional`. Also, the feature `custom_time` takes
precedence over `js-sys` so these do not actually conflict and one
_could_ enable both.

Author: Frederik Rothenberger

identity_core/Cargo.toml

@@ -21,8 +21,8 @@ time = { version = "0.3.23", default-features = false, features = ["std", "serde
 url = { version = "2.4", default-features = false, features = ["serde"] }
 zeroize = { version = "1.6", default-features = false }
 
-[target.'cfg(all(target_arch = "wasm32", not(target_os = "wasi"), not(feature = "custom_time")))'.dependencies]
-js-sys = { version = "0.3.55", default-features = false }
+[target.'cfg(all(target_arch = "wasm32", not(target_os = "wasi")))'.dependencies]
+js-sys = { version = "0.3.55", default-features = false, optional = true }
 
 [dev-dependencies]
 proptest = { version = "1.0.0" }