Ability to create the default page model

For controllers and templates, the default one can be overridden by a custom one.
This is not the case for page models, which could be useful to ensure site consistency.
See https://github.com/getkirby/ideas/issues/556

This can be solved using the provided solution plus a pageMethod for every model function to ensure the methods are available on any page object, but it feels a bit weird to write a model function by function

Edit: site/models/default.php is only loaded when the exact template name is default; not when default is a fallback template/model.

This PR can be inspired:

https://github.com/getkirby/kirby/pull/3747