Allow mv multiple files at once

[PlasmaIntec]

Description

#6102

Turn the required "source" argument into a rest "source(s)" argument, then iterate across sources using .flat_map().

Tests

Make sure you've done the following:

• Add tests that cover your changes, either in the command examples, the crate/tests folder, or in the /tests folder.
• Try to think about corner cases and various ways how your changes could break. Cover them with tests.
• If adding tests is not possible, please document in the PR body a minimal example with steps on how to reproduce so one can verify your change works.

Make sure you've run and fixed any issues with these commands:

• cargo fmt --all -- --check to check standard code formatting (cargo fmt --all applies these changes)
• cargo clippy --workspace --features=extra -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect to check that you're using the standard code style
• cargo test --workspace --features=extra to check that all the tests pass

[sam701]

@PlasmaIntec Don't you mind to change the cp behavior, as well, to keep them consistent? It would be great if mv and cp have the same "signature".

[fdncred]

thanks