Added `CalendarDelegate` class that supports plugging in custom calendar
logics other than Gregorian Calendar System.
Here is an example implementation for Nepali(Bikram Sambat) Calendar
System:
https://github.com/sarbagyastha/nepali_date_picker/blob/m3/lib/src/nepali_calendar_delegate.dart
Demo using the `NepaliDatePickerDelegate`:
https://date.sarbagyastha.com.np/
Fixes https://github.com/flutter/flutter/issues/77531,
https://github.com/flutter/flutter/issues/161873
## Pre-launch Checklist
- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide], including [Features
we expect every widget to implement].
- [x] I signed the [CLA].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] I followed the [breaking change policy] and added [Data Driven
Fixes] where supported.
- [x] All existing and new tests are passing.
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Co-authored-by: Tong Mu <dkwingsmt@users.noreply.github.com>
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