Add option to sort a Matrix element index

[bensomething]

Carrying on from the #16631 discussion (as that was considered a bug and resolved), please consider this a request to add the option to sort Matrix element indexes like you can regular element indexes. I've run into a few scenarios when this would be very handy.

• A list of performances, sorted by descending start date.
• A list of timeline items, sorted by date.
• An album track list, sorted by track number.

It'd be nice to not have to reorder these on the back-end, and instead have them display how they would on the front-end.

Examples

Performances

Timeline

Track List

[brandonkelly]

Made this possible for Craft 5.7 🎉 (#16732)

[bensomething]

You're the best. Thanks, Brandon!