Review attribute casing

[gadenbuie]

Would it make sense to you if createElement() inside of js/utils/_utils.ts replaced _ with - (similar to what htmltools in Python does)?

Originally posted by @cpsievert in #1853 (comment)

Relatedly, we should review and enforce:

• Snake case in R/Python, i.e. Tag("shiny-chat-message", content_type="html")
• Kebab-case in HTML, i.e. <shiny-chat-message content-type="html">
• camelCase in TS/JS, i.e. this.contentType.