Rollup merge of #99544 - dylni:expose-utf8lossy, r=Mark-Simulacrum

Expose `Utf8Lossy` as `Utf8Chunks`

This PR changes the feature for `Utf8Lossy` from `str_internals` to `utf8_lossy` and improves the API. This is done to eventually expose the API as stable.

Proposal: rust-lang/libs-team#54
Tracking Issue: #99543

library/std/src/lib.rs

@@ -258,6 +258,7 @@
 #![feature(staged_api)]
 #![feature(thread_local)]
 #![feature(try_blocks)]
+#![feature(utf8_chunks)]
 //
 // Library features (core):
 #![feature(array_error_internals)]