dub will now warn on unrecognized fields in `dub.json`, settings, or selections file

Previously, dub was silently accepting anything it didn't recognize
in `dub.json`, `[dub.]settings.json` and `dub.selections.json`. While the original
intent was to make forward-compatibility easy, it proved detrimental
as typos would just mean the user setting was ignored.

From this release, dub will now warn about any entry in its configuration files
or in `dub.selections.json`. After 10 releases, those warnings will turn into errors.