Initial commit: CleanPlate shared Dart package

Domain models, enums, constants, validators shared between frontend and backend.

.gitignore

@@ -0,0 +1,7 @@
+# https://dart.dev/guides/libraries/private-files
+# Created by `dart pub`
+.dart_tool/
+
+# Avoid committing pubspec.lock for library packages; see
+# https://dart.dev/guides/libraries/private-files#pubspeclock.
+pubspec.lock