New lint: unsafe trait becomes safe

[obi1kenobi]

This is the opposite direction of #231, and perhaps surprisingly this is also a breaking change since unsafe impl is not allowed for safe traits.

Implementing an unsafe trait requires an unsafe impl block, but safe traits cannot be implemented with an unsafe impl block. https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html#implementing-an-unsafe-trait

Needs schema additions to expose whether the trait is unsafe:
https://doc.rust-lang.org/nightly/nightly-rustc/rustdoc_json_types/struct.Trait.html
https://github.com/obi1kenobi/trustfall-rustdoc-adapter/blob/0b1f021c22aab6e48f38156e52c7b8331447149a/src/rustdoc_schema.graphql#L382