Rollup merge of #136355 - GuillaumeGomez:proc-macro_add_value_retrieval_methods, r=Amanieu

Add `*_value` methods to proc_macro lib

This is the implementation of https://github.com/rust-lang/libs-team/issues/459.

It allows to get the actual value (unescaped) of the different string literals.

Part of https://github.com/rust-lang/rust/issues/136652.

r? libs-api