Create explicit plugin contract

[abdala]

It transfers the plugin import/creation responsability to the user

export type Plugin = RequireAtLeastOne<{
  arrange(args: unknown, basePath: string, testRunnerArgs?: unknown): Promise<boolean>
  act(args: unknown, basePath: string, testRunnerArgs?: unknown): Promise<boolean>
  assert(args: unknown, basePath: string, testRunnerArgs?: unknown): Promise<boolean>
  clean(args: unknown, basePath: string, testRunnerArgs?: unknown): Promise<boolean>
}>