feat: additional --preview-interval values

[vmobilis]

Summary

Additional values for --preview-interval option of sd-cli:

• value 0 should preview only the last step of base-resolution sampling before the high-resolution pass.
  Can be used in --hires generations to save (in --preview-path) the base-resolution result together with the high-resolution final image.
  With feat: support numbering for preview images #1895, can save all base-resolution results of batch generations.
• values less than 0 should decode only the -Nth single image (to preview a sample and not to slow the generation down).

Additional Information

I have enabled "last step" preview for "high noise" video pass, but I didn't test any video generations and not sure if this is much useful.

The sd-server should be unaffected because it doesn't set the preview interval and uses default "every step" value.

Checklist

• I have read and confirmed this PR follows the contribution guidelines.