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.

docs/source/FlatBuffers.md

@@ -4,7 +4,7 @@ FlatBuffers    {#flatbuffers_index}
 # Overview {#flatbuffers_overview}
 
 [FlatBuffers](@ref flatbuffers_overview) is an efficient cross platform
-serialization library for C++, C#, C, Go, Java, JavaScript, Lobster, Lua, TypeScript, PHP, Python, and Rust.
+serialization library for C++, C#, C, Go, Java, Kotlin, JavaScript, Lobster, Lua, TypeScript, PHP, Python, and Rust.
 It was originally created at Google for game development and other
 performance-critical applications.
 
@@ -51,7 +51,7 @@ under the Apache license, v2 (see LICENSE.txt).
     needed (faster and more memory efficient than other JSON
     parsers).
 
-    Java and Go code supports object-reuse. C# has efficient struct based
+    Java, Kotlin and Go code supports object-reuse. C# has efficient struct based
     accessors.
 
 -   **Cross platform code with no dependencies** - C++ code will work
@@ -108,7 +108,7 @@ sections provide a more in-depth usage guide.
     present for every object instance.
 
 -   Use `flatc` (the FlatBuffer compiler) to generate a C++ header (or
-    Java/C#/Go/Python.. classes) with helper classes to access and construct
+    Java/Kotlin/C#/Go/Python.. classes) with helper classes to access and construct
     serialized data. This header (say `mydata_generated.h`) only depends on
     `flatbuffers.h`, which defines the core functionality.
 
@@ -132,6 +132,8 @@ sections provide a more in-depth usage guide.
     own programs.
 -   How to [use the generated Java/C# code](@ref flatbuffers_guide_use_java_c-sharp)
     in your own programs.
+-   How to [use the generated Kotlin code](@ref flatbuffers_guide_use_kotlin)
+        in your own programs.
 -   How to [use the generated Go code](@ref flatbuffers_guide_use_go) in your
     own programs.
 -   How to [use the generated Lua code](@ref flatbuffers_guide_use_lua) in your