Fix dynamic function return type extension for get_sites()

[IanDelMar]

This PR

• fixes the dynamic function return type extension for get_sites(),
• adds correct tests,
• bumps the required minimum version of PHPStan to 1.10.0 (needed for Type::getConstantScalarValues()),
• removes deprecated instanceof.

What has been fixed?

• There is no $args['fields'] = 'count' but $args['fields'] = 'ids' and $args['count']. The extension now checks for both $args['fields] and $args['count']. Not properly checking for 'count' led to incorrect return types for truthy count types.
• A return type for indetermined argument type has been added.

Todo:

• Adding tests for string parameter.

[szepeviktor]

Thank you Ian.