Rollup merge of #120577 - wutchzone:slice_split_at_unchecked, r=m-ou-se

Stabilize slice_split_at_unchecked

Greetings!

I took the opportunity, and I tried to stabilize the `slice_split_at_unchecked` feature. I followed the guidelines, and I hope everything was done correctly 🤞 .

Closes #76014

Author: matthiaskrgr