feat(packages): Add scaling and anchoring to the background command

[Omikhleia]

New options: page (for multi-page images such as a PDF), scale (for disabling scaling), preserveaspect (for aspect ratio preservation when scaling), anchor (for positioning of the image when needed).

Moreover a color and an image src can both be set, for cases where the image has transparent areas or isn't scaled to the full bleed area (due to aspect ratio preservation or absence of scaling).

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The default behavior so far for images (corresponding to scale=true, preserveaspect=false) was nice in many cases (e.g. when using some old-paper-like texture on pages)...

But the extended proposal here is something I considered for some projects:

• In one, I fancied having some sort of floral ornaments at the top-right corner of chapter pages (i.e. ideally anchor=ne, scale=true, preserveaspect=true to avoid distortion if changing the book paper size but using the same image file)
• In another, I fancied having some markings (e.g. draft, confidential etc.) on pages (so anchor=center, scale=false).
• When using a PDF as "background image", the ability to pick the page to use is also a nice option (e.g. in the just mentioned case for markings, using a PDF allows for scalable drawings, and one can have all backgrounds in a given file, etc.)

This is but a proposal, which I find interesting for my needs, but it also raises some good questions:

• Whether allpages should be true by default...
• Whether allpages and one-page backgrounds should be stacked rather that one disabling the other... On that matter see also in-code comment on the tricky question for the user for controlling where page breaks occur. That would lead to questioning the command(s) and their scope...
• Whether the page (+ bleed) area is always the target, or some specific frame id could be used... But this needs to be carefully considered, I played with it in another experiment but didn't include it here; it can quickly become a hornets' nest (due to switching masters and layouts etc.) and it is then hard to explain with good sense...

Feedbacks are welcome.