Fix ko.observable handling in Linq translations

[exyi]

• FirstOrDefault, LastOrDefault, ... should behave like indexers
  • return observable, if the array contains observables, which then needs to be unwrapped
  • this is a regression in 4.3
• Where, Order, Take, ... return the same structure which the original array has
• Select is weird as it unwraps all observables in the array, but not recursively. That's why patch adds the JsObjectObservableMap object, which can represent this mid-state (and many other combinations)
  • Indexing after Select was apparently always broken in DotVVM