suggestion: simplify plugin middleware creation

[deer]

We currently have:

export interface PluginMiddleware<State = Record<string, unknown>> {
  /** A path in the format of a filename path without filetype */
  path: string;

  middleware: Middleware<State>;
}
export interface Middleware<State = Record<string, unknown>> {
  handler: MiddlewareHandler<State> | MiddlewareHandler<State>[];
}

This is a bit clunky for plugin creators, due to the following syntax:

{
      middleware: {
        handler: twoPointlessMiddlewares,
      },
      path: "lots-of-middleware",
}

Ideally they should just specify the handler and path:

{
      handler: twoPointlessMiddlewares,
      path: "lots-of-middleware",
}

So this suggests a type like the following:

export interface PluginMiddleware<State = Record<string, unknown>> {
  /** A path in the format of a filename path without filetype */
  path: string;
  middleware?: Middleware<State>;
  handler?: MiddlewareHandler<State> | MiddlewareHandler<State>[];
}

While this makes sense now, the downside of this is that if we ever want to add anything to Middleware, then just specifying a handler and path in plugins is no longer valid. Thoughts on this concern, or this suggestion in general? @bjesuiter

from: #1545 (comment)