Exit `wp-custom-header.js` gracefully in browsers without `addEventListener` support.

[peterwilsoncc]

wp-custom-header.js throws an error in browsers that don't support document.addEventListener.

Add a check for browser support and fail gracefully if it doesn't pass.

[peterwilsoncc]

38585.diff​ adds required check.

[peterwilsoncc]

In 39050:

Themes: Exit wp-custom-header.js early in browsers wihout modern events.

Adds a check for window.addEventListener support before attempting to use it. In browsers without support, the file is exited early to avoid throwing a JavaScript error.

Fixes #38585.