Feature Request: Add Nepali DatePicker (Bikram Sambat Calendar) Support

### Is there an existing issue for this?

- [ ] I have searched the existing issues

### Package/Plugin version

1.0.0

### What you'd like to happen

```Markdown
### Description
I would like to request the addition of a Nepali DatePicker widget to the `flutter_form_builder` package. This widget should support the Bikram Sambat (BS) calendar, which is the official calendar of Nepal and widely used in the country.

### Justification
- The addition of a Nepali DatePicker will significantly benefit developers and users in Nepal who use the `flutter_form_builder` package.
- It will enhance the localization capabilities of the package and make it more inclusive for users from different cultural backgrounds.

### Expected Behavior
- A new `FormBuilderNepaliDatePicker` widget that works similarly to `FormBuilderDateTimePicker` but supports the BS calendar.
- The widget should provide options to disable past and future dates, set min and max dates, and choose the date format.
- The widget should handle the conversion between AD (Gregorian) and BS (Nepali) dates internally.
```

### Alternatives you've considered

https://pub.dev/packages/flutter_form_builder_nepali_date_picker/

### Aditional information

_No response_