Add proposal "Format API"

[mickael-menu]

This proposal introduces a dedicated API to easily figure out a file format.

While a Publication is independent of any particular format, knowing the format of a publication file is necessary to:

• determine the publication parser to use,
• group or search publications by file type in the user's bookshelf.

This API is not tied to Publication, so it can be used as a general purpose tool to guess a file format, e.g. during HTTP requests or in the LCP library.

You can read the formatted proposal here.

[mickael-menu]

@danielweck @jccr We would like to merge this proposal during the next Readium dev call, so any insights you may have for desktop and web is most welcomed 🙏

[mickael-menu]

I'd like to move forward, so I intend to merge this proposal after next week's meeting, if there's no counter-arguments till then.

I made a few changes in the proposal after implementing it in Swift:

• Renamed Format.guess into Format.of, because the caller is expecting an accurate Format returned.
• Added bitmap formats sniffing, because we need them in the CBZ parser.
• Removed inspectingContent parameter to simplify the sniffers. Instead, Format.of() will iterate twice through the sniffers: first with a context containing only file extensions and media types, and the second time with a context containing the content, if there's any.
• I grouped the sniffing of several formats in single sniffers, when the logic is shared (e.g. all the Readium WebPub formats).
• Added a few more APIs:
  • MediaType: encoding, structuredSyntaxSuffix, isZIP, isJSON, isRWPM, isPartOf()
  • Link: mediaType,
  • Format.SnifferContext: encoding,contentAsRWPM and readFileSignature() to sniff magic numbers.