config printing enhancements

rustfmt currently provides the ability to print configuration details in various forms:

• default (documented briefly here) which dumps the full set of configuration options with their default values, regardless any existing detected or specified configuration inputs
• current - dumps the configuration incorporating specified and detected values (e.g. any additionally included command line flags, detected configuration files, etc.)
• minimal - the subset of the configurations from current that are actively utilized when formatting some specified input code/file (the primary use case for this is to help users locate unused/inoperable options and to be able to remove them from their rustfmt config file)

in #6247 there was some discussion about potential enhancements to this current configuration printing feature, and I'd suggest there's two potential (non mutually exclusive) items we could consider:

• show the full set of configuration options with the default values based on the specified style edition
• show the subset of configuration options that changed for a specified style edition

Start by reviewing rustfmt's existing configuration-printing behavior described in README.md and the discussion in issue #6247. Define the desired edition-aware output for default and/or changed options, then verify that the selected enhancement is implemented and covered by the project's relevant tests.