Set aria-* attributes to help screen readers when using occlusion

When ember-table occludes table rows, screen readers can have difficulty understanding the table since all the rows are not present in the DOM.

We can help them out by providing a few aria attributes:

• Adding aria-rowcount to the <table> element
• Adding aria-rowindex to each <tr>

This helps the screenreader understand the total row count and which rows are currently visible.

There's some info about this in https://sarahmhigley.com/writing/grids-part2/#lazy-loading-rows-or-columns

The vertical-collection addon used to do the occlusion looks like it provides the item index so I don't think this would be too difficult to add.

I started to implement a WIP of this but then I realized that I'm not sure how this needs to work to support the tree features So I need to look a little bit more into that.

Start by reviewing ember-table's occlusion behavior and the linked tree features documentation, then inspect how the vertical-collection addon exposes item indexes. Confirm how row indexes should work for both regular and tree rows. Done means the table exposes the total row count and each rendered row exposes its correct index without breaking tree behavior.