dart: use enhanced enums (#8313)

* dart: rename enums.fbs

* feat: use dart enhanced enums

* generate code

---------

Co-authored-by: Wouter van Oortmerssen <aardappel@gmail.com>

dart/test/enums.fbs

@@ -0,0 +1,10 @@
+enum OptionsEnum : uint32
+{
+    A = 1,
+    B = 2,
+    C = 3
+}
+
+table MyTable {
+    options : [OptionsEnum];
+}