js-on-sx: optional chaining ?.

Parser: jp-parse-postfix handles op "?." followed by ident / [ / (
emitting (js-optchain-member obj name), (js-optchain-index obj k),
or (js-optchain-call callee args).

Transpile: each emits (js-optchain-get obj key) or (js-optchain-call
fn args).

Runtime: js-optchain-get and js-optchain-call short-circuit to
js-undefined when receiver is null/undefined.

423/425 unit (+5), 148/148 slice unchanged.

lib/js/parser.sx

@@ -593,6 +593,42 @@
             (jp-call-args-loop st args)
             (jp-expect! st "punct" ")")
             (jp-parse-postfix st (list (quote js-call) left args)))))
+      ((jp-at? st "op" "?.")
+        (do
+          (jp-advance! st)
+          (cond
+            ((jp-at? st "punct" "[")
+              (begin
+                (jp-advance! st)
+                (let
+                  ((k (jp-parse-assignment st)))
+                  (jp-expect! st "punct" "]")
+                  (jp-parse-postfix
+                    st
+                    (list (quote js-optchain-index) left k)))))
+            ((jp-at? st "punct" "(")
+              (begin
+                (jp-advance! st)
+                (let
+                  ((args (list)))
+                  (jp-call-args-loop st args)
+                  (jp-expect! st "punct" ")")
+                  (jp-parse-postfix
+                    st
+                    (list (quote js-optchain-call) left args)))))
+            (else
+              (let
+                ((t (jp-peek st)))
+                (if
+                  (or
+                    (= (get t :type) "ident")
+                    (= (get t :type) "keyword"))
+                  (do
+                    (jp-advance! st)
+                    (jp-parse-postfix
+                      st
+                      (list (quote js-optchain-member) left (get t :value))))
+                  (error "expected ident, [ or ( after ?.")))))))
       ((or (jp-at? st "op" "++") (jp-at? st "op" "--"))
         (let
           ((op (get (jp-peek st) :value)))