Add `Parallel` method

### Prototype
```cs
public static TOut[] Parallel(this T @this, params Func toRun)
=> Task.WaitAll(toRun.Select(Task.Run));
```

### Comments
There also should be an `Action`-based overload.

Also, a method which would run async lambdas and async actions.

### Unresolved questions
What's the name for the method?

How do we not confuse async lambdas with lambdas returning a task?

No files, tests, or entry points are named. Start by locating HonkSharp's existing extension methods and reviewing the proposed Task.WaitAll and Task.Run usage; completion requires resolving the method name and defining distinct overload behavior for synchronous functions, actions, async lambdas, and async actions.

Written by the indexing model from the issue text.