jQuery DataTables references the standard CharacterData interface which
was undefined in obscura's DOM environment, causing ReferenceError and
empty page renders.
- Add CharacterData class extending Node with data/length/substringData/
appendData/insertData/deleteData/replaceData
- Make Text and Comment extend CharacterData instead of Node
- Update _wrap() to create proper Text/Comment instances by nodeType
- Add create_comment_node op for Document.createComment()
- Fix set_text_content to handle Comment node mutations
Fixes#72
Co-authored-by: KuaaMU <xcm853629353@gmail.com>