Customize: Restore ability to add Custom Link nav menu items for jump links...

Customize: Restore ability to add Custom Link nav menu items for jump links and other URLs that were previously allowed.

Simplify regular expression for checking URL validity to just do basic checks to confirm the value looks like a URL. Leave the complete validation to the server-side logic in `WP_Customize_Nav_Menu_Item_Setting::sanitize()` to avoid having to maintain two separate codebases for validating URLs.

Props westonruter, SergeyBiryukov for testing.
Amends [41697].
See #32816.
Fixes #42506 for trunk.

Built from https://develop.svn.wordpress.org/trunk@42153


git-svn-id: https://core.svn.wordpress.org/trunk@41984 1a063a9b-81f0-0310-95a4-ce76da25c4cd