Add missing std feature to allow to be used as dependency of proc-macro

Author: GuillaumeGomez

Cargo.toml

@@ -5,3 +5,9 @@ edition = "2021"
 description = "Provides code to unescape string literals"
 license = "Apache-2.0 OR MIT"
 repository = "https://github.com/rust-lang/literal-escaper"
+
+[dependencies]
+std = { version = '1.0.0', optional = true, package = 'rustc-std-workspace-std' }
+
+[features]
+rustc-dep-of-std = ["dep:std"]