Reinstate JPEG export

[jgadsden]

Describe what problem your feature request solves:
The feature to export diagrams as JPEG was removed because version 2.18.1 of the AntV/x6 package did not work for JPEGs - it exported as PNG instead of JPEG
It would be good to reinstate the ability to export as JPEG when this library is fixed

Describe the solution you'd like:
Reinstate the ability to export as JPEG along with the existing SVG and PNG.

Additional context:
The JPEG feature was removed in issue "JPEG actually exports PNG" #1319
Use the linked pull request #1324 as a guide to reinstate this feature

[jgadsden]

No news yet of when this feature may be fixed in AntV/x6 although there has been some activity in the issue:
antvis/X6#4083