Stabilize feature `char_indices_offset`
Stabilized API:
```rust
impl CharIndices<'_> {
pub fn offset(&self) -> usize;
}
```
Tracking issue: https://github.com/rust-lang/rust/issues/83871
Closes https://github.com/rust-lang/rust/issues/83871
I also attempted to improved the documentation to make it more clear that it returns the offset of the character that will be returned by the next call to `next()`.
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