I18N: Harden against undefined index in `_load_script_textdomain_from_src()`.

This guards against an undefined index warning being raised when a script or script module is registered with a URL that lacks a path component. 

This also adds full PHPStan type definitions for `wp_parse_url()`, ensuring that the `_load_script_textdomain_from_src()` function has no PHPStan errors at rule level 10.

Developed in https://github.com/WordPress/wordpress-develop/pull/11690

Follow-up to r62278.

Props westonruter, manzoorwanijk, mukesh27.
See #65015, #64238.


git-svn-id: https://develop.svn.wordpress.org/trunk@62293 602fd350-edb4-49c9-b593-d223f7449a82