Support template for WP_Widget settings

[lipemat]

The WP_Widget class supports passing an "$instance" array around which contains the value of the current widget settings.

Without a PHPStan template, adding a type to the $instance array results in warnings like "should be contravariant with parameter" for the following methods:

• form
• update
• widget

Having a template available allows using @extends to pass the array shape around.

As a bonus, this pull request also includes the full shape of the $args passed to WP_Widget::widget