[Feature] Add maxUnit to prettyDuration

[akurnaz]

Hi,

The number of units can be fixed using "tersity" and "upperTersity", but it would be nice if it was determined by "upperTersity".

For example:

prettyDuration(
          const Duration(seconds: 5),
          abbreviated: true,
          tersity: DurationTersity.second,
          upperTersity: DurationTersity.week,
          //maxUnit: 2,
)

Current: 5s
Expectation: 5s

prettyDuration(
          const Duration(hours: 8, minutes: 3, seconds: 5),
          abbreviated: true,
          tersity: DurationTersity.second,
          upperTersity: DurationTersity.week,
          //maxUnit: 2,
)

Current: 8h, 3mins, 5s
Expectation: 8h, 3mins

prettyDuration(
          const Duration(days: 10, hours: 8, minutes: 3, seconds: 5),
          abbreviated: true,
          tersity: DurationTersity.second,
          upperTersity: DurationTersity.week,
          //maxUnit: 2,
)

Current: 1w, 3d, 8h, 3mins, 5s
Expectation: 1w, 3d