Update for Dart version upgrade and updated dependencies.

Update Dart. Update dependency.

v2.1.2 for publishing

Update for version 2.1.1 — just changelog updates for 2.1.0

Version 2.1.0 updated for Dart 3.x.

 - Support for Vinculum or Apostrophus (both compact with Unicode

   and tally marks)
- **NOTE**: this change means the maximum value of common/default config is
     3,999; 399,999 for 'compact' apostrophus', and 3,999,999 for both
     tally mark apostrophus and vinculum styles.
 - Housekeeping for bigger Dart update, as well as `test` and `analyze`
   cleanup.
 - Improved code coverage report generation to be "pure Dart" and scripted.

Version 1.1.1

Allow user to specify placeholder character for nulla/zero.