Currently only one type: ParsingDuration, which handles both
integer-nanoseconds and time.ParseDuration-compatible strings. This
makes it compatible with any existing JSON config files, while expanding
support for human readable strings.

Also, move the close-brace for the empty Decoder struct onto the same
line as the open-brace.

Integrate the new jsontypes.ParsingDuration type into the cue and json
decoders via the `SingleTypeSubstitutionMangler`.