[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.fbs

@@ -0,0 +1,14 @@
+
+enum ABC: int { void, where, stackalloc }
+
+enum public: int { }
+
+table KeywordsInTable {
+  is: ABC = void;
+  private: public;
+}
+
+union KeywordsInUnion {
+  static: KeywordsInTable,
+  internal: string
+}