Add WC_Abstract_Order::is_editable()

To centralise logic used to determine if an order can be edited from
the Edit Order screen
Add 'wc_order_is_editable' filter
Store order's editable status in property

To save overhead of a function call
Merge pull request #6154 from thenbrent/is_editable

Add `WC_Abstract_Order::is_editable()` method and `'wc_order_is_editable'` filter