Fix (create mutations for uncovered classes): adjust regex to ignore classnames which are prefixed with a classname to create mutations for

[CaptainRedbeard24]

Given following classes:

final class User implements UserInterface
{
    ...
}

final class UserCollection implements CollectionInterface
{
    ...
}

and a testfile:

covers(User::Class);
...

Currently the plugin would create mutations for both classes, even though the UserCollection class does not have any tests. This is due to the Regex in \Pest\Mutate\Support\MutationGenerator::doesNotContainClassToMutate /(?:class|trait)\\s+$class.*/. The $class.* will match everything that starts with the name of the target class.

This PR adds a word boundary after $class to prevent that.