Include reactor information in comment reaction information

[alexr00]

Currently, the emoji reactions on the comments widget just shows the number of reactions, not any info about where those reactions came from:

On GitHub, for example, the reactions include the usernames of the reactors in hover text:

We could expand the comments reaction API to allow something similar by adding a reactors property to the CommentReaction:

	export interface CommentReaction {
		readonly reactors?: readonly string[];
	}

To show something like this: