Allow limited access to OsStr bytes

[epage]

OsStr has historically kept its implementation details private out of
concern for locking us into a specific encoding on Windows.

This is an alternative to #95290 which proposed specifying the encoding on Windows. Instead, this
only specifies that for cross-platform code, OsStr's encoding is a superset of UTF-8 and defines
rules for safely interacting with it

At minimum, this can greatly simplify the os_str_bytes crate and every
arg parser that interacts with OsStr directly (which is most of those
that support invalid UTF-8).

Tracking issue: #111544