Script Loader: Disable injectPolyfill flag in webpack config.

This is a follow-up to [57981] to ensure that `wp-polyfill` is no longer automatically added as a dependency to package scripts. This avoids unnecessarily loading the polyfill script in places such as the block editor.

See #60962.

git-svn-id: https://develop.svn.wordpress.org/trunk@58162 602fd350-edb4-49c9-b593-d223f7449a82

Author: swissspidy