message: support creating pills in partcreator
diff: @@ -260,6 +260,13 @@ class RoomPillPart extends PillPart {
  }
  }
  +class AtRoomPillPart extends RoomPillPart {
  + get type() {
  + return "at-room-pill";
  + }
  +}
  +
  +
  class UserPillPart extends PillPart {
  constructor(userId, displayName, member) {
  super(userId, displayName);
  @@ -402,6 +409,10 @@ export class PartCreator {
  return new RoomPillPart(alias, room);
  }
  + atRoomPill(text) {
  + return new AtRoomPillPart(text, this._room);
  + }
  +
  userPill(displayName, userId) {
  const member = this._room.getMember(userId);
  return new UserPillPart(userId, displayName, member);
  