fix: use getExecOutput() and set outputs in finally block

[yxxhero]

Summary

Fixes #218

• Replace manual listeners with getExecOutput() from @actions/exec, which uses StringDecoder to properly handle multi-byte character splits across Buffer boundaries (e.g. ANSI color codes from helm-diff output)
• Move setOutput calls to a finally block so that exit-code, helmfile-stdout, and helmfile-stderr are always set — even when the command fails to spawn or throws unexpectedly

How to test

- name: Run helmfile
  id: helmfile_diff
  uses: helmfile/helmfile-action@fix/empty-outputs-218
  with:
    helmfile-args: diff --environment staging
- name: Check outputs
  run: |
    echo "exit-code: ${{ steps.helmfile_diff.outputs.exit-code }}"
    echo "stdout length: $(echo '${{ steps.helmfile_diff.outputs.helmfile-stdout }}' | wc -c)"
    echo "stderr length: $(echo '${{ steps.helmfile_diff.outputs.helmfile-stderr }}' | wc -c)"