chore: plan the JS formatter

js/readme.md sets out the port of bun's JS formatter onto oxc's parser:
a layer of handles with bun's API over node tables built in one walk,
how bun's tree maps onto oxc's, the comment pass, the entry points for
files and template code, and the choices: JSDoc formatting on, import
sorting an option, no Flow. Cargo.toml adds oxc 0.156 behind a js
feature, on by default.