Modify the Dart enum generation to match other usage of c_style_enum

[jerel]

Summary

The C# implementation of enum generation looks at all variants of an enum and, if all are of type Unit, it then generates a native enum and otherwise generates a class. The current Dart implementation simply generates all classes if c_style_enums = false or all enums if c_style_enums = true.

This PR changes the Dart logic to match C# (the only other language that currently implements the c_style_enums config. dart.rs will now generate all classes if c_style_enums = false and if it is true it will generate native Dart enums unless the enum contains data, then it will generate a class.

Test Plan

A test was added that verifies that when c_style_enums is enabled the generator will generate native Dart enums for the simple case but generate classes for data-containing enums.

[ma2bd]

Thanks for the PR. Looks reasonable to me

[ma2bd]

Just released under serde-generate 0.24.0

[jerel]

Excellent, thanks for this!