add headless reference package

[felixroos]

the doc.json is the basis for the reference. it is used in the reference sidebar, the docs, the autocomplete and the tooltips. it would be useful if it could be used via a package, e.g. to implement an autocomplete or a custom reference elsewhere (e.g. nudel).
so far, the codemirror autocomplete and tooltip are not exported from the @strudel/codemirror package, because the doc.json file is almost half a megabyte large. i wouldn't add that to the codemirror package by default, so maybe there should be a @strudel/reference package that exports the doc.json + maybe some helpers to work with it.