feat(Text): Support formatting and emojis in fromAttributed

[absidue]

This pull request refactors the handling of attributed text to add support for formatting (bold, italics, strikethrough) and emojis (custom ones and unicode ones). To avoid breaking changes, I took the approach of translating the new response as closely as possible to the existing classes and properties, so for anyone with existing code to handle TextRuns and EmojiRuns this should just work.

This currently only handles the styleRuns, commandRuns and attachmentRuns, however adding support for the decoratedRuns (only seen them used to add a grey "pill" around some links in video descriptions), shouldn't be too difficult with the way this is designed.

Examples:

• https://www.youtube.com/watch?v=OyVK4M7sgDE (video description has formatting, as well as links to other YouTube pages and external pages).
• https://www.youtube.com/watch?v=OqiXFXlYFi8 (comments have various combinations of bold, italics and strikethrough)
• https://www.youtube.com/watch?v=v3wm83zoSSY (comments have custom emojis)

To test the two comment use cases, you will need to use #614 too, so that it can handle the new comment nodes.

P.S. The way YouTube's attributed text works seems very similar to Apple's NSAttributedString/NSMutableAttributedString classes, so it seems likely that this may have originally been designed for the iOS client.