message: update IMigrationTool.volunteerForMigration() return type
diff: @@ -53,7 +53,7 @@ export interface IMigrationTool extends IEventProvider<IMigrationToolEvents> {
  * @returns A promise which resolves when the local client has been selected to perform the migration. The
  * migration may have already been completed prior to being selected.
  */
  - volunteerForMigration(): Promise<void>;
  + volunteerForMigration(): Promise<boolean>;
  /**
  * Whether the local client is selected to perform the migration.
  */
  