Add lint that all crates are #[unstable]

[brson]

It's easy to forget to write it when adding new crates, as in #31120, and then release a crate as stable accidentally.

In tidy.py add a whitelist of crates that can be stable; in the loop, if the file name looks like 'src/*/lib.rs' it should have the attribute unless its on the whitelist.

[alexcrichton]

👍

[brson]

Since #31120 also was adding a single attribute everywhere, the lint might additionally look for other common crate attributes.

[brson]

I can mentor.

[KiChjang]

Can I pick this up? I've been meaning to understand the business of lints.