Description

Fixes #6270. It's once again possible to run a batch script named foo.bat or foo.cmd by typing foo without the file extension.

External command execution on Windows now works like this:

1. Try to run the external command as a normal executable
2. If that fails:
   1. Check whether the command name matches a known cmd.exe builtin. If so, run it.
   2. [New in this PR] Check whether which-rs can find an executable in PATH or the cwd. which-rs has logic to include executable file extensions in its search, so it will find foo.cmd given just foo. Run the found executable if its file name differs from the one we already attempted to run.

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