Make `ConfigCache` public

[blaugold]

I'm maintaining a package (daco) that expands formatting of Dart files to include doc comments. Doc comments are formatted as markdown with prettier and embedded Dart code blocks with the Dart formatter. daco provides an executable of the same name that is meant to be used as a drop-in replacement for dart format, so it also formats the actual Dart code of the files it is used on.

I want to respect the formatting preferences of the directory in which the tool is run, and ConfigCache would be very helpful for that.

[munificent]

I would like to expose some sort of API so that code using dart_style as a package can use the same configuration file system lookup logic that dart format uses. But I don't want to simply make ConfigCache public because it wasn't really designed to be public API. I'd want to make some changes to it first.