Add subxt feature in subxt-signer crate to default features (#1193)

* add subxt to subxt-signer default features

* distinguish between native and web in signer

* fix formatting and clippy

* rustfmt

signer/src/lib.rs

@@ -38,3 +38,6 @@ pub use secrecy::{ExposeSecret, SecretString};
 // SecretUri's can be parsed from strings and used to generate key pairs.
 // DeriveJunctions are the "path" part of these SecretUris.
 pub use crypto::{DeriveJunction, SecretUri, SecretUriError, DEV_PHRASE};
+
+#[cfg(all(feature = "subxt", not(any(feature = "web", feature = "native"))))]
+compile_error!("subxt-signer: When using the 'subxt' feature, exactly one of the 'web' and 'native' features should be used.");