Refactor feed slices (#4834)

* Copy FeedViewPost into FeedSliceItem

* Explicitly construct feed slice items by copying known fields

* Type rootItem as FeedViewPost for now

Mergefeed logic relies on that.

* Unify reason and __source for slice items

* Move feedContext out of FeedSliceItem

* Remove slice.isFlattenedReply

* Remove unnused slice.ts

* Inline slice.isFullThread

* Refactor condition for clarity

* Extract slice.includesThreadRoot

* Encapsulate more usages of slice.rootItem into slice

* Rename slice.rootItem so semi-private slice._feedPost

* Move reason into slice

* Simplify slice ctor argument

* Reorder getters to reduce diff

* Make feedContext a getter to reduce diff

src/lib/api/feed/merge.ts

@@ -251,7 +251,7 @@ class MergeFeedSource_Following extends MergeFeedSource {
       dryRun: false,
       maintainOrder: true,
     })
-    res.data.feed = slices.map(slice => slice.rootItem)
+    res.data.feed = slices.map(slice => slice._feedPost)
     return res
   }
 }