Bangle_setUI_F18: add btnRelease (read below)

Explicitly state button should fire on falling edge.

Does not change `btn` mapped handlers to act on rising edge just jet.

As per suggestion in
espruino#2571 (comment).

libs/js/banglejs/Bangle_setUI_F18.js

@@ -69,7 +69,7 @@
       Bangle.swipeHandler = options.swipe;
       Bangle.on("swipe", Bangle.swipeHandler);
     }
-    if (options.btn) {
+    if (options.btn || options.btnRelease) {
       Bangle.btnWatches = [
         setWatch(function() { options.btn(1); }, BTN1, {repeat:1,edge:"falling"}),
         setWatch(function() { options.btn(2); }, BTN2, {repeat:1,edge:"falling"}),