Add exists? in CLJS

[borkdude]

Rudimentary version, cljs.analyzer should be replaced with SCI specifics.

(defn ^:macro exists?*
  "Return true if argument exists, analogous to usage of typeof operator
   in JavaScript."
  [_ _ x]
  (if (symbol? x)
    (let [x     (cond-> (str x) #_(:name (cljs.analyzer/resolve-var &env x))
                  (= "js" (namespace x)) name)
          segs  (str/split (str (str/replace (str x) "/" ".")) #"\.")
          n     (count segs)
          syms  (map
                 #(vary-meta (symbol "js" (str/join "." %))
                             assoc :cljs.analyzer/no-resolve true)
                 (reverse (take n (iterate butlast segs))))
          js    (str/join " && " (repeat n "(typeof ~{} !== 'undefined')"))]
      (list 'boolean (concat (list 'js* js) syms)))
    `(some? ~x)))