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>

include/flatbuffers/util.h

@@ -703,6 +703,8 @@ enum class Case {
   kDasher = 7,
   // THEQuiCKBr_ownFox (or whatever you want, we won't change it)
   kKeep = 8,
+  // the_quick_brown_fox123 (as opposed to the_quick_brown_fox_123)
+  kSnake2 = 9,
 };
 
 // Convert the `input` string of case `input_case` to the specified `output_case`.