Add basic Kotlin support (#5409)

* [Kotlin] Add kotlin generate code for tests and add
kotlin test to TestAll.sh

* [Kotlin] Add Kotlin generator

This change adds support for generating Kotlin classes.

The approach of this generator is to keep it as close
as possible to the java generator for now, in order
to keep the change simple.

It uses the already implemented java runtime,
so we don't support cross-platform nor js Kotlin yet.

Kotlin tests are just a copy of the java tests.

* Add optional ident support for CodeWriter

Identation is important for some languages and
different projects have different ways of ident
code, e.g. tabs vs spaces, so we are adding optional
support on CodeWriter for identation.

* [Kotlin] Add Documentation for Kotlin

* [Kotlin] Modify generated code to use experimental Unsigned types.

include/flatbuffers/code_generators.h

@@ -26,7 +26,7 @@ namespace flatbuffers {
 // Utility class to assist in generating code through use of text templates.
 //
 // Example code:
-//   CodeWriter code;
+//   CodeWriter code("\t");
 //   code.SetValue("NAME", "Foo");
 //   code += "void {{NAME}}() { printf("%s", "{{NAME}}"); }";
 //   code.SetValue("NAME", "Bar");
@@ -38,7 +38,8 @@ namespace flatbuffers {
 //  void Bar() { printf("%s", "Bar"); }
 class CodeWriter {
  public:
-  CodeWriter() {}
+  CodeWriter(std::string pad = std::string())
+      : pad_(pad), cur_ident_lvl_(0), ignore_ident_(false) {}
 
   // Clears the current "written" code.
   void Clear() {
@@ -67,9 +68,22 @@ class CodeWriter {
   // Returns the current contents of the CodeWriter as a std::string.
   std::string ToString() const { return stream_.str(); }
 
+  // Increase ident level for writing code
+  void IncrementIdentLevel() { cur_ident_lvl_++; }
+  // Decrease ident level for writing code
+  void DecrementIdentLevel() {
+    if (cur_ident_lvl_) cur_ident_lvl_--;
+  }
+
  private:
   std::map<std::string, std::string> value_map_;
   std::stringstream stream_;
+  std::string pad_;
+  int cur_ident_lvl_;
+  bool ignore_ident_;
+
+  // Add ident padding (tab or space) based on ident level
+  void AppendIdent(std::stringstream &stream);
 };
 
 class BaseGenerator {