DevTools custom formatter test

Exercises Js_of_ocaml_devtools.register_formatters (library js_of_ocaml.devtools). Requires a compiler built with Introcaml; the formatter is a no-op under wasm.

To see the formatter in the console: open DevTools, enable Settings > Preferences > Console > Enable custom formatters, then log the sample values and expand them in the console. The values are also logged once on load.

Mode: —

Rendering of the formatter output

What the console would show: the header of each value, expandable to the entries of its body (rendered lazily, as the console does).