Support internal trait when building synthetic enum trait

[JordonPhillips]

When converting the normal enum trait to an enum shape, we convert the "internal" tag into the @internal trait. This updates the opposite direction conversion used to generate the synthetic enum trait to add the "internal" tag if the member had the @internal trait if it isn't already in the list of tags.

I also added a few other test cases for the member -> enum trait definition test that didn't seem to be covered, at least from the perspective of an enum shape that wasn't itself generated by converting a string shape.

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