Add ability to scroll picker preview

[Doodler8888]

The new git picker for staging (Space g s) already allows to scroll preview on Alt-<arrow_down>/<arrow_up>. It would be nice to be able to scroll the preview area while using other pickers like the one for searching files.

[Frost-Phoenix]

That would be super useful. I suggested it as part of #512, a PR has been created for it (#542) but was closed soon after.

There is already a scroll system for popups, like <space>i for vsc changes or shell commands output. Maybe we could re-use that for previews as well.

About scrolling in git picker, it is strange that is uses a separate system. Also the preview doesn't have a scrollbar even when it is scroll-able. And the <A-up> and <A-down> binds are not super convenient since they are quite far from the home row. Maybe we could use something like <A-j> and <A-k> as a more ergonomic alternative. Those are the binds that where proposed in helix-editor/helix#15854.

What do you thinkg @matoous ?