Custom Order Admin UI

Patch Adds

• Switched registers from config entities to content entities to match how stores are done. Since there should be 1 or more registers per store, this makes sense to be content not config.
• Removed custom registerFloat option and sets it as part of the baseDefinition, also set it to commerce_price type and relabeled it "cash" since it should take the cash in the register at any time, not just the opening float. Open to name changes since "cash" isn't a great name either.
• Added store ID to registers so they track what store they belong to.
• Update RegisterListBuilder to not show machine name and instead show parent Store.
• Added missing order_items config for POS order type, as it is required and orders can't be added without it. This wasn't clear, should probably add some documentation about it. mglaman says it is for views compatibility reasons.
• Added Registers class to get list of available registers
• Added initial POS form to select active register and set in session.
• Added POS form for the "main" section, which is building orders and checking out, which is based off of the ContentEntityForm for the order object.
• Added empty CSS file to be used for main POS styling
• Added twig template file for main POS form page