API for editing and deleting document comments

[RMacfarlane]

Extensions should be able to handle edits and deletes to comments within the editor. The DocumentCommentProvider can be extended to add these new methods, mimicking the other methods on the provider.

	interface DocumentCommentProvider {
		provideDocumentComments(document: TextDocument, token: CancellationToken): Promise<CommentInfo>;
		createNewCommentThread(document: TextDocument, range: Range, text: string, token: CancellationToken): Promise<CommentThread>;
		replyToCommentThread(document: TextDocument, range: Range, commentThread: CommentThread, text: string, token: CancellationToken): Promise<CommentThread>;

		/**
		 * Called when a user edits the comment body to the be new text text.
		 */
		editComment?(document: TextDocument, comment: Comment, text: string, token: CancellationToken): Promise<Comment>;

		/**
		 * Called when a user deletes the comment.
		 */
		deleteComment?(document: TextDocument, comment: Comment, token: CancellationToken): Promise<void>;

		onDidChangeCommentThreads?: Event<CommentThreadChangedEvent>;
	}

Additionally, comments themselves need a flag indicating whether or not they can be edited or deleted.

	interface Comment {
		<existing properties>
		/**
		 * Whether the current user has permission to edit the comment.
		 *
		 * This will be treated as false if the comment is provided by a `WorkspaceCommentProvider`, or
		 * if it is provided by a `DocumentCommentProvider` and  no `editComment` method is given.
		 */
		canEdit?: boolean;

		/**
		 * Whether the current user has permission to delete the comment.
		 *
		 * This will be treated as false if the comment is provided by a `WorkspaceCommentProvider`, or
		 * if it is provided by a `DocumentCommentProvider` and  no `deleteComment` method is given.
		 */
		canDelete?: boolean;

	}