Specify closure this for extend

[pindab0ter]

What:

• Bug Fix
• New Feature

Description:

When extending expect(), we are met with phpstan: Undefined variable: $this. This is resolved by specifying what $this will refer to in the closure.

Example of what this PR fixes:

expect()->extend('toBeTrashed', function () {
    Assert::assertThat(
        $this->value, // phpstan: Undefined variable: $this
        new class () extends Constraint {
            public function matches(mixed $other): bool
            {
                if (!($other instanceof Model && method_exists($other, 'trashed'))) {
                    throw new InvalidArgumentException('Can only be used on Models, not on queries.');
                }

                return $other->trashed();
            }

            public function toString(): string
            {
                return 'is trashed';
            }
        }
    );

    return $this; // phpstan: Undefined variable: $this
});