[JSC] Allow indexed module namespace object fields

[JSC] Allow indexed module namespace object fields
https://bugs.webkit.org/show_bug.cgi?id=168870

Reviewed by Saam Barati.

JSTests:

* wasm/spec-tests/names.wast.js:

Source/JavaScriptCore:

While JS modules cannot expose any indexed bindings,
Wasm modules can expose them. However, module namespace
object currently does not support indexed properties.
This patch allows module namespace objects to offer
indexed binding accesses.

* runtime/JSModuleNamespaceObject.cpp:
(JSC::JSModuleNamespaceObject::getOwnPropertySlotCommon):
(JSC::JSModuleNamespaceObject::getOwnPropertySlot):
(JSC::JSModuleNamespaceObject::getOwnPropertySlotByIndex):
* runtime/JSModuleNamespaceObject.h:

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@213453 268f45cc-cd09-0410-ab3c-d52691b4dbfc