js-on-sx: spread ... in array literals and call args

Parser: jp-array-loop and jp-call-args-loop detect punct "..."
and emit (js-spread inner).

Transpile: when any element is spread, build array/args via
js-array-spread-build with (list "js-value" v) and (list
"js-spread" xs) tags.

Runtime: js-array-spread-build walks items, appending values or
splicing spreads via js-iterable-to-list (handles list/string/dict).

Works in arrays, call args, variadic fns (Math.max(...arr)),
and string spread ([...'abc']).

414/416 unit (+5), 148/148 slice unchanged.

lib/js/runtime.sx

@@ -1750,6 +1750,26 @@
 
 (define Object {:entries js-object-entries :values js-object-values :freeze js-object-freeze :assign js-object-assign :keys js-object-keys})
 
+(define
+  js-array-spread-build
+  (fn
+    (&rest items)
+    (let
+      ((result (list)))
+      (for-each
+        (fn
+          (item)
+          (let
+            ((kind (nth item 0)))
+            (cond
+              ((= kind "js-spread")
+                (for-each
+                  (fn (x) (append! result x))
+                  (js-iterable-to-list (nth item 1))))
+              (else (append! result (nth item 1))))))
+        items)
+      result)))
+
 (define js-array-is-array (fn (v) (list? v)))
 
 (define js-array-of (fn (&rest args) args))