a11y: render LDP container contents as <ul>, not a 2-column <table>

[m5x5]

Summary

• Every row in a folder listing has the same predicate (ldp:contains), so the markup is a list of resources, not a data table. Screen readers now announce "list, N items" instead of an opaque table.
• mainTable is now <ul class="folder-listing"> with aria-label="Contents of <folder>". Each entry is a <li> wrapping the outliner's object cell (passed elementName='div' so it emits a <div>, not a stray <td> outside any table context).
• Package branch (container holding an index.ttl): host element is <div class="folderPanePackageContents"> instead of <table> — rendering is delegated to GotoSubject and no row layout is needed.
• CSS: drop .folderPaneMainTable / td / th / .folderPanePredicateCell rules; add .folder-listing / .folder-listing__item rules.

Addresses SolidOS/profile-pane#310.

Coordinated PRs

• a11y: promote .app-view to the page-level <main> landmark mashlib#394 — Promote .app-view to the page-level <main> landmark
• a11y: demote tab-widget body from <main> to <div> solid-ui#747 — Demote tab-widget body from <main> to <div>
• a11y: drop inner <main> from ProfileView and EditProfileView profile-pane#312 — Drop inner <main> from ProfileView and EditProfileView
• a11y: replace table-as-layout markup with semantic elements solid-panes#640 — Replace table-as-layout markup in outline & data-content panes

[m5x5]

I think we could remove the comments I added but besides that it looks good from my side.