Create a commerce_condition plugin type, replacing commerce_promotion_condition

We currently have promotion conditions, but other entity types (payment gateways, shipping methods) need it as well, so we need to move them into the base commerce module. We also want to make sure they're completely divorced from core conditions.
We looked into core conditions and how both core and rules use them, and decided that we don't want to use Context, since it imposes a DX penalty and doesn't match our use case (where we always have a single entity provided, while the configuration form does the rest, unlike rules where every setting is a context). Having our own plugin type will also help us build the custom condition UI for which we're wrapping up the design.