generate-webview-js: add comment concerning core-js
generate-webview-js: add comment concerning core-js

Add explanatory comment concerning the observed and documented
behavior of `core-js`, and why we don't enable it, despite how easy it
would be.

vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile,vishwesh3/zulip-mobile