cdk proposal: framework for rich text editors ("draftjs" for angular)

tl;dr https://draftjs.org/ for angular

I've been searching the internets and there is nothing that comes even close to draftjs, which is built on top and requires react. What does it do? It solves most of the [contenteditable issues](https://medium.engineering/why-contenteditable-is-terrible-122d8a40e480) and provides api for people to build their own rich text editors on top of it.

Rich text editor doesn't necessarily mean WYSIWYG textarea, it can help with anything from mentions, hashtags to replacing text emoticons with images etc.


Some of the most popular draftjs plugins can be found [here](https://www.draft-js-plugins.com/) and [here](https://github.com/nikgraf/awesome-draft-js), it should give you an idea what can be done with it.

worth reading:
- https://code.facebook.com/posts/1684092755205505/facebook-open-sources-rich-text-editor-framework-draft-js/
- https://medium.com/@rajaraodv/why-draft-js-and-why-you-should-contribute-460c4a69e6c8
- https://medium.com/@rajaraodv/how-draft-js-represents-rich-text-data-eeabb5f25cf2

I think this could be a great fit for CDK, given that you are eventually planning on creating WYSIWYG anyway.. what do you think?

@jelbourn @mmalerba @kara @andrewseguin @DevVersion @crisbeto

まず、issue に記載されている Draft.js のリソースと Angular CDK のコンテキストを確認します。リンクされているエディターおよびプラグインのリファレンスも含めてください。実装の前に、提案する Angular 統合、API の範囲、受け入れ基準を定義します。フレームワーク設計とプロジェクトへの適合について合意できた時点で作業は完了です。

索引モデルが issue の本文から書いたものです。