2.0.0: feat: ensure passed tags are strings

Newer versions of PHP raise a warning if you attempt to use array syntax
against non-strings / non-arrays. Added sanity check to ensure we're
working with a string, otherwise skip it.

Also added in PHP 8.1 to the testing suite and bumped the copyright.

Bumped version to 2.0 since as the change in expected output is a
breaking change.