Loosely based on https://phabricator.services.mozilla.com/D25101
Includes changes to `GetEmojiPresentation`, added in e38b57ca.
The `EmojiComponent` response includes hair color, skin tone, ZWJ, VS16, Combining Enclosing Keycap, and the Tag unicode characters used for subdivision flag emojis.
This means that `IsClusterExtender` should support all these characters, and be forward compatible with any future Unicode changes.
This also lets us restructure `ClusterIterator::Next` to check for EmojiComponents after a TextDefault character, without any stupid constants.
Vim control lines were re-introduced or not entirely cleaned up.
This nukes them again.
Removing from the rest of js, caps, chrome, config, devtools, docshell,
image, intl. More to come.
