Add sniff to disallow import use statements for functions/constants in the global namespace

[jrfnl]

Discussed on Slack with @herregroen

Disallow import use statements for functions and constants from the global namespace:

use func esc_attr;

use const PHP_VERSION_ID;

in favour of enforcing fully qualified names in the inline code:

\esc_attr( $something );

\version_compare( \PHP_VERSION_ID, '70300' );

[jrfnl]

I'm going to close this issue as "fixed".

Importing functions/constants is now disallowed. The use of fully qualified global functions and constants is now enforced, so this should be considered covered.