Rollup merge of #124678 - UserIsntAvailable:feat/stabilize-split-at-checked, r=jhpratt

Stabilize `split_at_checked`

Closes #119128

For the const version of `slice::split_at_mut_checked`, I'm reusing the `const_slice_split_at_mut` feature flag (#101804). I don't if it okay to reuse tracking issues or if it preferred to create new ones...

Author: compiler-errors