Rollup merge of #134016 - zachs18:stable-const-str-split_at, r=Amanieu

Stabilize `const_is_char_boundary` and `const_str_split_at`.

Tracking issues: #131516, #131518

Stabilized const API:

```rs
// in `core`
impl str {
    // const_is_char_boundary feature
    const fn is_char_boundary(&self, index: usize) -> bool;

    // const_str_split_at feature, depends on const_is_char_boundary
    const fn split_at(&self, mid: usize) -> (&str, &str);
    const fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str);
    const fn split_at_checked(&self, mid: usize) -> Option<(&str, &str)>;
    const fn split_at_mut_checked(&mut self, mid: usize) -> Option<(&mut str, &mut str)>;
}
```

This will allow safely splitting string slices during const-eval.

Closes #131516, Closes #131518

This will need FCP.
r? libs-api

IIUC these do not use any new const language features (i.e. they are implementable manually on stable 1.83.0 using `unsafe`: [playground link](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=3679632cd1041084796241b7ac8edfbd)).

Cc ``@rust-lang/wg-const-eval`` (I don't know if I have the permissions for this ping; if not, someone else please ping wg-const-eval if it is necessary)

Author: workingjubilee