allow user custom group_imports order

[Sherlock-Holo]

currently, the group_imports option only accepts Preserve and StdExternalCrate. If I want a new order like StdCrateExternal, it need to PR the rustfmt, make it supports this order

consider the group_imports can accept a new value type: string array, like: ["Std", "External", "Crate"], it will allow users custom their project import order, like ["Std", "Crate", "External"]

[calebcartwright]

Thanks for reaching out with your suggestion. This has been brought up before, but I'm still not convinced it's a particularly viable path to begin with.

If anyone feels strongly enough about this and is willing to come up with a PR that deals with both the complexity of open ended free form user entry and mapping/transforming that's generalized to support sorting that free form (or adding the logic to handle all the variant combinations) then I'll certainly review and consider.