Add Atom feed generator as a new package

[myl7]

Changes

• Add a new package astro-atom that works like astro-rss to generate Atom 1.0 feed (aka. RFC 4287)
• Deprecated "Passing a glob result directly" is not added
• Atom-specified attributes are not added, but the attribute names are changed to fit the ones in Atom feed. Maybe need to add a transformer for the two attribute sets.

Related #1657, #1113, proposals/0020-api-route-signature (mentioned)

I am not sure if a new proposal is required for this new package, since it expands the code base to be managed.
Anyway, I will first finish the left parts shown below to make this a complete work.

Testing

• Tests like the ones in astro-rss are added correspondingly, except the ones in pagesGlobToRssItems.test.js. To add the later ones soon.

Docs

To be added soon, and it should be mostly similar to the one in astro-rss

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