Dart generate keywords (#4449)

* Add script to use Dart SDK to generate keywords

* Read generated keywords in dart generator

bin/utils/dart-keywords/save-dart-keywords.dart

@@ -0,0 +1,16 @@
+import 'dart:io';
+
+import 'package:analyzer/dart/ast/token.dart';
+
+main() {
+  // save keywords to a text file
+  final txtFile = File(
+      '../../../modules/openapi-generator/src/main/resources/dart/dart-keywords.txt');
+  String txtString = '';
+  for (String keyword in Keyword.keywords.keys.toList()) {
+    txtString += keyword + '\n';
+  }
+  txtString =
+      txtString.substring(0, txtString.length - 1); // remove last newline
+  txtFile.writeAsStringSync(txtString);
+}