Add [Dart] support (#4676)

* Add [Dart] support

* fix enum vectors

* Allow for opt out of string interning

* fix comment style, make interning opt in

* remove Offset<T>, prefer int

* avoid creating unnecessary vtable objects

* start work on tests - do not generate builder if struct has 0 fields - add int64

* support reading structs properly

* correctly handle reading vectors of structs, dartfmt

* support structs, fix unnecessary prepares

* fix bool customizations

* undo unintentional removal of file

* docs updates, complete tutorial, bug fix for codegen

* more documentation

* Update docs, add to doxygen file

* update package structure, add samples script/code

* rearrange sample

* Tests

* Add readme for pub

* cleanup package for pub

* update docs for renamed file

* remove custom matcher, use `closeTo` instead

* remove unintentional file

* remove unintended file checkin

* use auto, move method, cleanup

* refactor to ObjectBuilders, add Builders

* Update tests, examples

* Add files missing from previous commit

* documentation and example updates

* Update LICENSE, make dartanalyzer happy, fix minor bugs, get rid of duplicate files, publish script

* fix sample for slightly different schema

* Update pubspec.yaml

docs/source/doxygen_layout.xml

@@ -39,6 +39,8 @@
           title="Use in PHP"/>
       <tab type="user" url="@ref flatbuffers_guide_use_python"
           title="Use in Python"/>
+      <tab type="user" url="@ref flatbuffers_guide_use_dart"
+          title="Use in Dart"/>
       <tab type="user" url="@ref flexbuffers"
           title="Schema-less version"/>
       <tab type="usergroup" url="" title="gRPC">