[Feature Request] foreach loop / reducer

It would be great if there was some way to loop over lists so that we can process things like "SEGS" (from Impact Pack) one item at a time. For example, I can detect all the faces an image and then output that to a list of masks. For each mask I want to upscale/enhance that part of the image and paste it into the image.

I was thinking about how this would look in the UI, and I realized this is exactly like a "reducer" in many programming languages. It could look like this:

(The mockup in the screenshot is just a shell of a node)

On the left I have some node that produces a list, doesn't matter what, and that would go into the `array` input. `item` would output the first entry from the list/array, which I could process however I please, but it would most likely be merged into `accum` (accumulator), which would most likely be an output image (e.g. paste the enhanced face back into the image). Then you take that output and feed it back into the input `accum` of the `Reduce` node, thus forming a loop. `Reduce` then runs again with the next item in the input `array` and the updated `accum`, which repeats until the list is done, and then outputs the final result on `final`. `init` is the initial value for the output `accum`, for the first iteration of the loop. It's optional and will be `None` if not specified, just like [a normal reduce function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/reduce).

I would implement this myself as a custom node, but as far as I know I can't output/return more than once during a single "Queue Prompt" cycle like this node needs to do.

Start with the reducer mockup and the Queue Prompt execution behavior identified in the issue; no repository file or test is named. Done would mean list or array items can be processed sequentially, with an updated accumulator passed to the next iteration and a final result exposed when the list is complete.

Written by the indexing model from the issue text.