Detect scrolling up/down in amp-next-page trigger

### Description

Hi, I have this analytic trigger and I would like to include a new variable that shows if the page was scrolled up or down for the moment the analytic was triggered.
I need this variable because I am using infinite scroll (amp-next-page) and I would like to know if the page was accessed from the top (new page is coming) or back (user is going back)
`

{
"triggers": {
"trackScrollThrough": {
"on": "amp-next-page-scroll",
"request": "page",
"vars": {
"scrollType": "should be here up or down"
}
}
}

`

### Alternatives Considered

A new variable that shows the scroll' direction

### Additional Context

_No response_

Start by tracing the amp-next-page-scroll trigger through the amp-analytics and amp-next-page implementations. Determine how the trigger can expose whether the page was reached while scrolling up or down, then verify that the configured scrollType variable reports the direction at trigger time.

Written by the indexing model from the issue text.