Optionally have set-version update Cargo.lock

It would be useful in CI / automated publishing scenarios to have `cargo set-version ...` optionally (e.g. with a flag) able to bump the version in Cargo.lock.

Currently we're relying on subsequent commands (e.g. `cargo check`) in our workflow to bump the version in Cargo.lock, but this means that we can't run these commands with `--locked`.

Start at the `cargo set-version` command entry point and inspect how it currently updates the package version. Trace how the command handles the lockfile, then determine where an optional flag should apply. Done means the flag updates `Cargo.lock` while preserving the existing behavior when it is omitted, with coverage for both cases.

Written by the indexing model from the issue text.