rustdoc-json: Include safety of `static`s
`static`s in an `extern` block can have an associated safety annotation ["because there is nothing guaranteeing that the bit pattern at the static’s memory is valid for the type it is declared with"](https://doc.rust-lang.org/reference/items/external-blocks.html#statics). Rustdoc already knows this and displays in for HTML. This PR also includes it in JSON.
Inspired by https://github.com/obi1kenobi/cargo-semver-checks/issues/975 which needs this, but it's probably useful in other places.
r? `@GuillaumeGomez.` Possibly easier to review commit-by-commit.
08f1f28699