Multiple commands support on Windows

Related to https://github.com/buildkite/agent/issues/1538 but aimed at coverage and consistency with other platforms (or clear documentation about it not being supported).

On Windows, executing the step below only shows the output with the version once, it completes without executing the second command from the list.

```
commands:
- buildkite-agent --version
- buildkite-agent --version
```
During execution, the first command exits with no errors so the expected behaviour is to see the second command run.

More broadly, some types of command item such as an executable, may cause the generated wrapper script for Windows jobs to exit in a non-intuitive way due to the semantics of the batch script. Please, make this more consistent with the intuitive behaviour on other platforms.

Reproduce the Windows job using the commands list and inspect the generated wrapper script, especially how executable command items affect batch-script execution. Done means both commands run consistently with other platforms, or the unsupported behavior is clearly documented; verify using the provided buildkite-agent --version example.

Written by the indexing model from the issue text.