Namerdart (#7187)

* Apply Namer to Dart.

- Also refactor idl_gen_dart a bit
  - to use more const and references
  - out parameters should be the last argument

* Add keyword test

* minor fixes

* fix merge

* extra 's'

* move dart keyord into dart dir

* Address comments

* Use $ for escaping keywords
* Outparameters for namespace_map

* Escape dollar in toString

* Escape dollar in toString2

* Use UpperCamelCase for types and variants

* try to fix ToString

* namer Type fixes

* Remove path prefixing in imports

* gen code

Co-authored-by: Casper Neo <cneo@google.com>

tests/keyword_test.fbs

@@ -8,6 +8,7 @@ table KeywordsInTable {
   is: ABC = void;
   private: public;
   type: int;
+  default: bool = false;
 }
 
 union KeywordsInUnion {