Add a flag for keeping target directory on clean

[nandesu-utils]

implements #11887, --keep-directory flag for cargo clean

[rustbot]

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[ehuss]

The team discussed this change and have decided that this isn't something we want to add. This is a highly specialized flag, which is something we generally try to avoid. Additionally, PRs are generally only reviewed for accepted features. Before posting a PR for a new feature, usually we want it to be designed and accepted (through an issue or RFC). Will follow up with more discussion on #11887.