Merge commerce_combine_carts into Commerce Core as option

Problem/Motivation

Thank you very much for this super important module. I think multiple carts for the same user is very untypical and should be offered as setting (opt-in?) by the shop system. Having one cart is what the users expects and is used to from any other Shop system.

So I think it would make a lot of sense to move the functionality of this contrib module into Drupal Commerce Core (Cart / Checkout) and make it a setting?

The usage statistics also show it's used by many Drupal Commerce websites.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes