feat(cdk/drag-drop): Proposal for Keyboard and Screenreader Accessibility

### Feature Description

Hello everyone,

Accessibility is nowadays a very important topic and therefore I think it is essential to get as much work as possible done directly in the framework. Therefore is my proposal to make the Angular CDK Drag and Drop accessible to everyone! My ideas so far

- Reusing as much code as possible (maybe renaming the functions if selected)
- Adding aria roles if necessary
- tabindex to all grabbable items
- Space to activate moving by keyboard (then activly moving by arrow keys with small movement). Further ideas/improvement: movement to fix containers or movement by bigger (user defined) steps!
- Escape to quit movement
- Implementation according to https://www.w3.org/wiki/PF/ARIA/BestPractices/DragDrop

First Code proposal (Please don't look to specific, is my first proposal, Improvements needed):
https://github.com/angular/components/commit/1c50475095a7ecf3c1f142c820d3f5a71f0070b8

Is is meaningful to do more work on it? Please be so kind and share your meanings with me!

Best regards,

Martin

### Use Case

Drag and Drop Accessibility!

Angular CDK Drag and Drop のエントリーポイントから始め、リンクされたコード提案を W3C ARIA DragDrop のベストプラクティスと併せて確認します。実装とテストの範囲を特定する前に、キーボード、スクリーンリーダー、フォーカス、移動について受け入れる動作を明確にします。完了とは、合意されたアクセシビリティ設計と、提案されたインタラクションの動作検証が済んでいることを意味します。

索引モデルが issue の本文から書いたものです。