[ViewModel] Implement accessibilityValue for slider

[CharlesMcD]

We should add a string that represents the current value of the accessibility element.

From the documentation

The value is a localized string that contains the current value of an element. For example, the value of a slider might be 9.5 or 35% and the value of a text field is the text it contains.

Use the value property only when an accessibility element can have a value that is not represented by its label. For example, a volume slider’s label might be “Volume,” but its value is the current volume level. In this case, it’s not enough for users to know the identity of the slider, because they also need to know its current value.