[C#] Handle keywords (#6775)

* Handle keywords in C#

C# should handle keywords similarly to other languages by appending an underscore after the keyword.

* Fix clang-format for idl_gen_csharp.cpp

* Fix spacing clang-format issue

tests/keyword_test/ABC.cs

@@ -0,0 +1,12 @@
+// <auto-generated>
+//  automatically generated by the FlatBuffers compiler, do not modify
+// </auto-generated>
+
+[Newtonsoft.Json.JsonConverter(typeof(Newtonsoft.Json.Converters.StringEnumConverter))]
+public enum ABC : int
+{
+  void_ = 0,
+  where = 1,
+  stackalloc_ = 2,
+};
+