Add first-class support for thiserror

[Bajix]

In a situation in which a function returns a thiserror error enum and a new error type is used, it would be extremely helpful if Rust Analyzer could recommend adding that error to that enum or otherwise creating a new error type. Done properly this could take the pain out of error type management

[Aloso]

You're in the wrong repo. This repo is only for rust-analyzer's website, not rust-analyzer itself. Please file this issue here.