feat(model): add named range targets

lib/src/names.dart

@@ -0,0 +1,19 @@
+import 'address.dart';
+import 'sheet.dart';
+
+sealed class NameTarget {
+  const NameTarget();
+}
+
+class CellName extends NameTarget {
+  final Sheet sheet;
+  final CellAddress address;
+  const CellName(this.sheet, this.address);
+}
+
+class RangeName extends NameTarget {
+  final Sheet sheet;
+  final CellAddress start;
+  final CellAddress end;
+  const RangeName(this.sheet, this.start, this.end);
+}