Introduce a new grammar renderer

This adds an extension to mdbook-spec that will parse code-blocks in a
BNF-style grammar into a rendered format, in both markdown or as
railroad diagrams.

Author: ehuss

mdbook-spec/Cargo.lock

@@ -5,6 +5,7 @@ edition = "2024"
 license = "MIT OR Apache-2.0"
 description = "An mdBook preprocessor to help with the Rust specification."
 repository = "https://github.com/rust-lang/spec/"
+default-run = "mdbook-spec"
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
@@ -15,6 +16,7 @@ once_cell = "1.19.0"
 pathdiff = "0.2.1"
 # Try to keep in sync with mdbook.
 pulldown-cmark = { version = "0.10.3", default-features = false }
+railroad = { version = "0.3.2", default-features = false }
 regex = "1.9.4"
 semver = "1.0.21"
 serde_json = "1.0.113"