Fixed position rows & parent/child row grouping support

[matuszewskijan]

Hey, thank you for the great library!

We've used it in our project but we were missing two features that I decided to vibe-code into the library. I'm not sure if anyone is interested in it so I didn't test it very extensively. If you see any value in those I'll be happy to improve the code and add some more tests.

Features:

• Sorting of nested tables, we've had a table with list of users per month, I've added a parent/child concept that keeps the nested rows sticked to their parent row, so the table still makes sense after sorting(without it you're getting the data completely mixed what makes no sense).

- Fixed position row: we wanted the `totals` row to stay at the top or bottom of the table always so we've added support for `data-position` attribute that keeps the row in the same place always.