How to make a script to select next track or previous track in the mix window?

Here you go:

If more than one track is selected the it will move the the leftmost track.

Move Left:

const sup = sf.ui.proTools
var allTracks = sup.trackGetVisibleTracks().names;
var allTracksCount = allTracks.length
var selTracks = sup.trackGetSelectedTracks().names;
var currentTrack = selTracks[0]
for (var i = 0; i < allTracksCount; i++) {
    if (allTracks[i].indexOf(currentTrack) == 0)
        break;
}
i = i-1;
if (i<0) {throw 0}
sup.trackSelectByName({
    names: [allTracks[i ]]
})

Move Right:

const sup = sf.ui.proTools
var allTracks = sup.trackGetVisibleTracks().names;
var allTracksCount = allTracks.length
var selTracks = sup.trackGetSelectedTracks().names;
var currentTrack = selTracks[0]
for (var i = 0; i < allTracksCount; i++) {
    if (allTracks[i].indexOf(currentTrack) == 0)
        break;
}
i = i + 1;
if (i > allTracksCount - 1) { throw 0 }
sup.trackSelectByName({
    names: [allTracks[i]]
});