feat: add zh-CN support

Author: kutear

lib/simple_moment.dart

@@ -20,4 +20,5 @@ export 'src/locales/nl.dart';
 export 'src/locales/vi.dart';
 export 'src/locales/km.dart';
 export 'src/locales/ko.dart';
+export 'src/locales/zh_CN.dart';
 export 'src/simple_moment_base.dart';

lib/src/locales/zh_CN.dart

@@ -0,0 +1,32 @@
+import '../localedata.dart';
+import '../identifier_position.dart';
+
+class LocaleZhCN extends ILocaleData {
+      final String _localeString;
+  String get localeString => _localeString;
+
+  LocaleEn([this._localeString = "zh-CN"]);
+
+  String get seconds => '几秒钟';
+
+  String get aMinute => '一分钟';
+  String get minutes => '%i分钟';
+
+  String get anHour => '一小时';
+  String get hours => '%i小时';
+
+  String get aDay => '一天';
+  String get days => '%i天';
+
+  String get aMonth => '一个月';
+  String get months => '%i月';
+
+  String get aYear => '一年';
+  String get years => '%i年';
+
+  String get futureIdentifier => '之后';
+  String get pastIdentifier => '之前';
+
+  IdentifierPosition get futurePosition => IdentifierPosition.prepend;
+  IdentifierPosition get pastPosition => IdentifierPosition.append;
+}