Implement Enhanced Enums feature in the CFE

[chloestefantsova]

This issue is for implementing dart-lang/language#158 in the CFE and referencing it in the related CLs.

See #47849 for the main tracking issue.

[devoncarew]

@chloestefantsova - did you mean this to be a general 'implement enhanced enums' issue, or just specific to the CFE? I started #47849 but then saw this issue.

[chloestefantsova]

@chloestefantsova - did you mean this to be a general 'implement enhanced enums' issue, or just specific to the CFE? I started #47849 but then saw this issue.

I meant this as the CFE specific one, so the two issues are in agreement with each other 🙂