Feature/add items minicart count type

[PaulEmich]

Short description and why it's useful

Some people prefer to show the total items in the cart, instead of the sum of the cart item quantities. This PR adds a config settings which makes you able to modify the cart count based on it's value.

Upgrade Notes and Changelog

• No upgrade steps required (100% backward compatibility)
• I've updated the Upgrade notes and Changelog on how to port existing VS sites with this new feature

Contribution and currently important rules acceptance

• I read and followed contribution rules
• I read the TypeScript Action Plan and adjusted my PR according to it
• I read about Vue Storefront Modules and I am aware that every new feature should be a module

[pkarw]

Thanks!

[patzick]

If we don't have issue for that let's add PR number to easily track changes:)
Just apply this suggestion to changelog and we'll merge.

Thanks for this feature!

[patzick]

I've added PR number resolving conflict. So it's all good now!

[pkarw]

Thanks!