[Feature]: `HasSingleItem()` add `.Item` to allow chaining on that item

### Problem Statement

It's possible to assign the `HasSingleItem()` to a variable, but it's not possible to chain on the value itself.

Like the `.Value` on Dictionaries #6185 , would be nice if the `HasSingleItem()` would allow chaining assertions on the item itself

### Proposed Solution

Add `.Item`

### Feature Category

Assertion

### How important is this feature to you?

QoL