Add Rust Playground metadata

derive_more is being pulled into the playground due to being a
dependency of some other crate in the top 100 crates, probably
<https://docs.rs/selectors>.

This adds metadata so that it gets installed with all features once
selectors updates to derive_more 2.0

Cargo.toml

@@ -40,6 +40,9 @@ rustc_version = { version = "0.4", optional = true }
 [badges]
 github = { repository = "JelteF/derive_more", workflow = "CI" }
 
+[package.metadata.playground]
+features = ["full", "std"]
+
 [features]
 nightly = []
 add_assign = []