Implement the ILibDurationFmt class to support DurationFormatting

[gouniLee]

Checklist

The following lists affects Pub Points on pub.dev when the package is published.

• Passed the all tests.
• Verified that the example app works.
• Executed the dart format command.
• Added the API description is added if necessary.

Description

Implement the new ILibDurationFmt class to support DurationFormatting.

• The tests for iLib's DurationFmt have been properly converted for Flutter and all have passed.
• The ILibDate class has been separated from the ilib_datefmt.dart file and moved to a new file.
• Re-geneate the assets/js/ilib-init.js file because scripts.json was missing. so the ilib-assemble tool code was modified to generate it.
  • ilib-assemble: Update the legacy ilib assembly to ensure that locale-independent root data is not omitted ilib-mono#245
• usage:

final ILibDurationFmtOptions fmtOptions = ILibDurationFmtOptions(locale: 'en-GB', length: 'full');
final ILibDurationFmt fmt = ILibDurationFmt(fmtOptions);
final ILibDateOptions dateOptions = ILibDateOptions(year: 1, month: 1, week: 1, day: 1, hour: 1, minute: 1);
fmt.format(dateOptions);

// '1 year, 1 month, 1 week, 1 day, 1 hour, 1 minute'