Add JPEG output format support

Summary

Add support for JPEG as an output format alongside the existing PNG, PDF, PS, and SVG outputs.

Context

The COMPARISON.md feature comparison identifies this as a gap between JairoSVG and EchoSVG.

Implementation Notes

Since JairoSVG already renders to a BufferedImage, adding JPEG output is straightforward:

1. Create a JpegSurface class (similar to PngSurface)
2. Use ImageIO.write(image, "JPEG", outputStream) in the finish() method
3. The BufferedImage type should be TYPE_INT_RGB (not ARGB) since JPEG does not support transparency
4. Add toJpeg() method to ConversionBuilder
5. Add jpeg to the CLI format options

This should be a small change — likely under 30 lines of new code.