Allow value enums to be variants of type enums

[worstell]

//ftl:enum
type TypeEnum interface {
  typeEnum()
}

type ValueEnumType ValueEnum
func (ValueEnumType) typeEnum() {}


//ftl:enum
type ValueEnum string

const (
  A ValueEnum = "a"
)

should be extracted as:

enum ValueEnum: String {
   A = "a"
} 

enum TypeEnum {
   ValueEnumType foo.ValueEnum
}

right now, it extracts as the following:

enum ValueEnum: String {
   A = "a"
} 

enum TypeEnum {
   ValueEnumType string
}

however, this pattern will remain erroneous:

//ftl:enum
type TypeEnum interface {
  typeEnum()
}

//ftl:enum
type ValueEnum string
func (ValueEnum) typeEnum() {}

const (
  A ValueEnum = "a"
)

because ValueEnum cannot be both a type variant name and itself a value enum. we do not support mixing of type and value variants in a single enum