Resolves #2766

Changed:
- Method `is_local()` to find occurrence on URL's hostname instead of the entire URL.
- Method `is_external()` to use `is_local()`.

This will resolve issues where the current host is a subdomain or substring of the URL's host.

Amends f73f5a2

Replaced with `str_starts_with()` to check for the double slash and `parse_url()` to check for the scheme.

This will resolve issues where `http` or `//` might be a substring of the URL's path.

if `wp_parse_url()` fails to extract or does not find a host name in the given URL parameter, presume that the parameter might be a host name itself, such as `example.org` (see unit test).