Add a collection formatter

[adpi2]

When inspecting a Scala (or Java) collection in the debugger, it would be very useful to see its elements as if it was an array (or rather a lazy array).

The end result should look like this:

myList: List[Int]
  size: 13
  elems
    0: "foo"
    1: "bar"
    ...
    10: "fizz"
    load more:
      11: "buzz"
      12: "bazz"

We can find some inspiration of how to do it in IntelliJ Scala debugger.

Corresponding feature request in Metals is: scalameta/metals-feature-requests#307