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

.gitignore

@@ -45,6 +45,7 @@ grpctest
 grpctest.exe
 snapshot.sh
 tags
+tests/dart_gen
 tests/go_gen
 tests/monsterdata_java_wire.mon
 tests/monsterdata_java_wire_sp.mon
@@ -94,3 +95,9 @@ js/flatbuffers.mjs
 build.ninja
 rules.ninja
 .vscode
+dart/.pub/
+dart/.packages
+dart/pubspec.lock
+dart/.dart_tool/
+dart/build/
+dart/doc/api/