[suggestion] Merge implementation of epub_write into epub about dart-epub

[orthros]

Hey @ShadowJonathan I actually did a fair amount of the merge between the two last week. If you check the lib/src/epub_writer.dart file and lib/src/writers directory, I tried to merge as much of the functionality as I could. I added a few integration tests for them as well.

Is this what you were looking for?

Thanks!

[ShadowJonathan]

I like it! I havent done a test on it yet (read a epub, then write it again, and compare differences), but so far it seems like a better version of what i managed to write!

[orthros]

I'm glad you like it! I didn't do much other than refactor and split a lot of what was originally in epub_write and added a few extra methods for processing more metadata, so thank you!

I wrote and added a few tests to the package that do round trip to ensure the content that is written can be re-read back. If you want to take a look I'd appreciate it!

[ShadowJonathan]

Will do!

Also I suggest making a "EpubFactory" class, since both EpubRef and EpubBook have "chapter entries" that are desynced from eachother, so a "Factory" class will be very useful when contructing a Epub, and being able to construct that to a legit and fully functional EpubBook that can be written with EpubWriter