notebook output scrollbar cannot be dragged if output continues to stream

[amunger]

1. create a notebook and run a cell that will continuously update over time

import time

for i in range(200):
    time.sleep(.3)
    print('string ' * 100)
    print(i)

2. try to drag the scrollbar while output is still being written
   🐛 scrollbar is not held and returns to the bottom

[amunger]

Streaming output items are all compressed into a single item in notebookCommon.compressOutputItemStreams (?), so we would need to stop doing that unless needed (for editing previous lines)

Also, the current RenderApi method only allows passing a single output item, but the output renderer will need to be able to render the entire output as it currently does, or just append the new data.

Current:

renderOutputItem(outputItem: OutputItem, element: HTMLElement, signal: AbortSignal): void | Promise<void>;

Proposed method that would only be used for streaming outputs and if the chosen renderer contains the method:

renderOutputItems?(outputItems: OutputItem[], edit: { offset: number, lastOutputId: string },  element: HTMLElement, signal: AbortSignal): void | Promise<void>;

Another alternative would be adding info to the OutputItem itself, containing only the appended string if appending should happen.