Add `*_value` methods to proc_macro lib
This is the (re-)implementation of https://github.com/rust-lang/libs-team/issues/459.
It allows to get the actual value (unescaped) of the different string literals.
It was originally done in https://github.com/rust-lang/rust/pull/136355 but it broke the artifacts build so we decided to move the crate to crates.io to go around this limitation.
Part of https://github.com/rust-lang/rust/issues/136652.
Considering this is a copy-paste of the originally approved PR, no need to go through the whole process again. \o/
r? `@Urgau`