Add support for audio and video tags in epub3 and add firstImageIsCover option

[uriesk]

Epub3 supports <video> and <audio> tags. This pull request adds support for them.
The media gets downloaded the same as images do.

Add firstImageIsCover option. When true, the first appearing image inside the ebook will get marked as cover in the manifest, so epub viewers can use it as thumbnail.

Caveats

• Audio and Video files get put into an audiovideo subfolder, which got recommended here.
• I change occuring controls attributes to controls="Controls", cause this is what XHTML wants. controls="controls" didn't work, cause rehype-stringify would change it back, but XHTML isn't case sensitive in attributes, so it works.
• an additional little commit, to not add <meta name="cover" content="image_cover"/ in the manifest if there is no cover (calibre ebook editor gave a warning for this).
• add start of content to guide
• additional ad nav landmarks to epub3, since <guide> is deprecated and at least Apple has the requirement for landmarks to exist.

[uriesk]

@e-adrien hi, i am now ready with this PR. I decided to add multiple commits for different issues to it, because it would be too cumbersome otherwise, as everything happens in only three files.

I did check with epubcheck-5.2.0 and with the check of calibre ebook editor, to make sure that everything is valid.

[e-adrien]

Hi @uriesk, thank you for this PR.

I think it will be better to use useFirstImageAsCover for the parameter name instead of firstImageIsCover.
I also think that the Audio and Video files embedding should be an optional feature (downloadAudioVideoFiles parameter, false by default).

Do you mind adding a test here ?

Thank you again for your work.

[e-adrien]

I wonder if we can avoid these if by puting some logic in this method

html-to-epub/src/index.ts

Line 693 in 6e68bcd

private async makeCover(): Promise<void> {

[uriesk]

@e-adrien added a bunch of test cases, fixed the formatting, renamed to useFirstImageAsCover and added downloadAudioVideoFiles

I don't like the long condition in the template either, but i didn't want to touch the makeCover stuff because it does so much extra things with creating a separate page for the cover.
Feel free to refactor it, but i personally think it's good how it is now.

Edit: the removing of the leading newline in that ibook file fixes an xml warning in calibres ebook editor