feat(paginator): expose previousPageIndex inside PageEvent

[crisbeto]

• Exposes the previous page index inside the object that is emitted by the page event.
• Reworks the page event tests to use a spy rather than saving the last result to the test component.

Fixes #10758.

[lacolaco]

I'm not sure it would be related to that, but the trigger which emits page event is not only a change of index but includes page size change.
How about previousPage: { size, index } ?

Or, just an idea, following core's SimpleChange interface;

{
  previousPage: { size, index },
  currentPage:  { size, index },
  firstChange: boolean
  isFirstChange(): boolean
}

[crisbeto]

@lacolaco I was trying to avoid a breaking API change. We can reconsider changing up the interface in the next major version.

[lacolaco]

@crisbeto You're right. I'm wondering ain't there a way to solve it without BCs. Thanks.

[andrewseguin]

I'm finding that many internal Google tests are failing because sometimes users create their own PageEvent (e.g. as an initial event on their pagination stream).

Would it make more sense to mark this field as optional to avoid a breaking change, as well as eliminate the need for such use cases to define a previous page index?

[crisbeto]

I've made it optional with a target to make it required in 7.0 @andrewseguin. Otherwise, I'm not sure whether we can eliminate the need for this property completely, e.g. I can see it coming in handy if we add the ability to jump to a page.

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