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/test.cpp

@@ -1416,6 +1416,8 @@ void UtilConvertCase() {
             "the_quick_brown_fox" },
           { "the_quick_brown_fox", flatbuffers::Case::kKeep,
             "the_quick_brown_fox" },
+          { "the_quick_brown_fox", flatbuffers::Case::kSnake2,
+            "the_quick_brown_fox" },
 
           // Tests for some snake_cases where the _ is oddly placed or missing.
           { "single", flatbuffers::Case::kUpperCamel, "Single" },
@@ -1468,6 +1470,12 @@ void UtilConvertCase() {
             flatbuffers::Case::kSnake, "abctest" },
           { flatbuffers::Case::kUpperCamel, "tHe_qUiCk_BrOwN_fOx",
             flatbuffers::Case::kKeep, "tHe_qUiCk_BrOwN_fOx" },
+          { flatbuffers::Case::kLowerCamel, "theQuick12345Fox",
+            flatbuffers::Case::kSnake, "the_quick_12345fox" },
+          { flatbuffers::Case::kLowerCamel, "a12b34c45",
+            flatbuffers::Case::kSnake, "a_12b_34c_45" },
+          { flatbuffers::Case::kLowerCamel, "a12b34c45",
+            flatbuffers::Case::kSnake2, "a12_b34_c45" },
         };
 
     for (auto &test_case : cases) {