Support to export fixed layout EPUBs

[spring-raining]

From Vivliostyle's Slack log:
https://vivliostyle.slack.com/archives/CAECW4S93/p1690301096707999

[thomas-topway-it]

@spring-raining great ! just a note. The export of fixed layout couldn't be managed by the Vivliostyle core itself ? This way it can be used both client side and from the command line (I would envisage for instance a client side use, because for the use within a MediaWiki extension, I want to avoid server side dependency)