CLJS-only projects do not have a CLJ entrypoint to enable RCF macroexpansion at macroexpanion time

One workaround is to use a shadow build-hook to load a CLJ namespace that sets `rcf/enable!`, see: https://clojurians.slack.com/archives/C7Q9GSHFV/p1688040682246669?thread_ts=1688031756.398229&cid=C7Q9GSHFV

See also #67

Example build hook: https://github.com/hyperfiddle/electric/blob/1f683adc6552be0f3f41b90e86305c6270917265/shadow-cljs.edn