Support for `start` to identify the bodymatter of a publication

[HadrienGardeur]

The EPUB spec offers the ability to indicate where a publication truly starts:

• using text in <guide> in EPUB 2.0.1: https://idpf.org/epub/20/spec/OPF_2.0.1_draft.htm#Section2.6
• using bodymatter in landmarks in EPUB 3.x: https://www.w3.org/TR/epub-ssv-11/#sec-partitions

This is by far the most commonly used landmark supported in EPUB, we can find it in ebooks from Standard Ebooks for example: https://github.com/standardebooks/nathaniel-hawthorne_the-house-of-the-seven-gables/blob/master/src/epub/toc.xhtml#L99

A common use case for this information is to open an ebook at the bodymatter instead of the first resource in readingOrder. This is something that Kindle apps and devices default to for example.

This is also useful for generating samples and has been used by Apple over the years: https://help.apple.com/itc/booksassetguide/en.lproj/itceb1fd1630.html

We would like to support this landmark in all Readium toolkits as well by using the start value in rel directly in the readingOrder.
We'll explore how we could support other landmarks in the future to extend this further, but for the time being, we can already focus on that one.

[mickael-menu]

Here's the Swift PR for reference: readium/swift-toolkit#602