[generate] If base type of enum is string, String() should return the original value

[neglect-yp]

// enumizer:target
type Color string

const (
    Red   Color = "red"
    Green Color = "green"
    Blue  Color = "blue"
)

If base type of enum is string like Color, String() should be as follows:

func String(c Color) string {
    switch c {
        case Red, Green, Blue:
            return string(c)
        default:
            return "unknown Color"
    }
}

If the value and String() result are different, users will be confused.