Add a WYSIWYG-esque editor for instructions

There are good pseudo-WYSIWYG editors (e.g. **bold text** is displayed bold etc.) for Markdown out there. A couple of good looking options are SimpleMDE and TOAST UI Editor, although both use CodeMirror rather than ACE (we use the latter for the main editors so it would be nice to share the dependency, although this is not a deal breaker).

Note that one thing we don’t want to do is load a Markdown editor library synchronously as part of the initial pageload—I think we will want to load it async and only if/when editing mode is activated. This might be a little tricky, especially before React releases support for async components, but I’m happy to share some ideas.

まず、既存のACEベースのメインエディタと手順編集フローを確認します。SimpleMDEとTOAST UI Editorを比較し、そのうえでエディタの動作と読み込み方針を定義します。手順でWYSIWYG形式のMarkdown編集体験を提供し、初回のページ読み込み中にMarkdownエディタを同期的に読み込まない状態になれば完了です。