First Draft: Nominal Sum Types via `enum union` and `switch` Expressions

[Meta]

On Monday, 14 September 2026 at 06:35:13 UTC, Meta wrote:

That first example should be:
```d
enum union ConfigValue
{
     case bool,
     case long,
     case double,
     case string,
     case string[],
}
```

The caseless version is a holdover from an earlier version that I 
forgot to change.