fix(occlusion): pass bufferSize based on scrollBuffer

[buschtoens]

Closes #485.

[buschtoens]

Looking at the actions made available by vertical-collection, this PR (in it's current form) doesn't really gain us anything yet. While the bufferSize is correctly calculated, the onScrolledToBottom action is not called yet.

Therefore I'll keep this WIP and also try to solve #508 in this PR.

[buschtoens]

Since we want to trigger onScrolledToBottom with scrollBuffer in mind, we should probably use lastVisibleChanged instead of lastReached.
Scratch that, turns out lastReached already accounts for bufferSize. 😊

onScroll could be shimmed with firstVisibleChanged. However the onScroll action wouldn't be called for every scroll event, but I think that's a worthwhile trade-off.

[buschtoens]

Okay, so I got the actions wired up, but we have a problem.

vertical-collection is not happy about the {{#if isLoading}} construct... Essentially Glimmer needs a static element to render the contents of {{#if isLoading}} into. If you wrap it with a plain <div>, everything works fine, but of course that bricks the layout...

There is a private {{#-in-element someElement}} helper that takes the block and renders it into someElement. This would fix our problem, however we need something like {{#-after-element someElement}}, which doesn't exist yet.

Semi-related issue: yapplabs/ember-tether#35 (comment)

[buschtoens]

While I'm trying to fix this (or get people to fix it 😜), we can still ship out a new version, since occlusion has to be enabled with a flag and tables with occlusion enabled and infinite scroll were essentially broken anyway. 😄

I'll drop a line in the release notes that instruct users to not use any conditionals in the the yielded block.

I'll just remove the {{yield}} block for now and add a line in the release notes about it. Seems safer and less troublesome for users.

Stuff like expanded-row also breaks v-c, since it has a top-level {{#if}} statement in its template.