Issue #1639 - Implement object.fromEntries()

Adds a self-hosted implementation of this map->object conversion.
This resolves #1639.

js/src/builtin/Object.cpp

@@ -1239,6 +1239,7 @@ static const JSFunctionSpec object_static_methods[] = {
     JS_FN("isFrozen",                  obj_isFrozen,                1, 0),
     JS_FN("seal",                      obj_seal,                    1, 0),
     JS_FN("isSealed",                  obj_isSealed,                1, 0),
+    JS_SELF_HOSTED_FN("fromEntries",   "ObjectFromEntries",         1, 0),
     JS_FS_END
 };