chore: markdown oracles

mdtidy's oracles run on tidy: the Prettier oracle through the Node
binding with embedded code off, as Prettier runs, and the shrink
oracle through the CLI with a generated config. The corpora build
into target/corpus/markdown, which survives reboots, and the
semantics oracle reads them by default. bench:ab times tidy's CLI.

Prettier oracle: A 871/872, B 2141/2142, C 3764/3781, D 301/308.