Add method that turns a Task into a Task<Unit>

[Mafii]

Name idea: VoidTaskToUnitTask or TaskToUnitTask

practical usages:

does not compile:

_ = test switch
{
    test is ISomeInterface some => await SomeMethodReturningTask(some),
    test is ISomeOtherInterface some => await SomeMethodReturningTask(some),
};

Would compile:

_ = test switch
{
    test is ISomeInterface some => await TaskToUnitTask(SomeMethodReturningTask(some)),
    test is ISomeOtherInterface some => await TaskToUnitTask(SomeMethodReturningTask(some)),
};

This feature could simplify some code and allow usage of switch expressions with await even when the Task is non-generic.

Currently you'd have to do this, which really is annoying:

var someTask = test switch
{
    test is ISomeInterface some => SomeMethodReturningTask(some),
    test is ISomeOtherInterface some => SomeMethodReturningTask(some),
};
await someTask;

[bash]

This should probably be called ActionToUnitAsync and for completeness we should also add UnitToActionAsync.

[bash]

We think improvements to C# should be preferred to this approach.
For instance Switch expression as a statement expression.