Azure Core linter rules for unions

There is plenty of union cases that are not nicely representable in many languages, I don't believe we have any linter rules today guiding here.

```ts
union A {
string,
{name: string}
}
```

The issue does not name files, tests, or an entry point. Start by locating the existing Azure Core linter rules and how union cases are tested, then clarify which union shapes need guidance, including the string-and-object example. Done means the agreed union patterns have linter rules with tests covering them.

Written by the indexing model from the issue text.