rustdoc-json: Support `unsafe_extern_blocks`

Followup to #126761 and #124482. rustdoc-json should be extended to support [RFC 3484](https://rust-lang.github.io/rfcs/3484-unsafe-extern-blocks.html).

This also intersects the [discussion of supporting unstable lang features in rustdoc-JSON](https://rust-lang.zulipchat.com/#narrow/stream/266220-t-rustdoc/topic/Rustdoc.20JSON.3A.20Experimental.20rustc.20features), which has no decision yet.