feat(tazjin/atom-feed): Add Nix functions for generating an Atom feed

This only adds the feed generation functions, but does not yet wire it
up to the blog content.

This was implemented against
https://validator.w3.org/feed/docs/atom.html and I've validated some
generated example feeds with the W3 validator.

Change-Id: Ide3ea90d3fa935047506aa87169100c2ead21284
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1709
Tested-by: BuildkiteCI
Reviewed-by: tazjin <mail@tazj.in>

users/tazjin/blog/fragments.nix

@@ -5,7 +5,8 @@
 # An entire post is rendered by `renderPost`, which assembles the
 # fragments together in a runCommand execution.
 #
-# The post index is generated by //web/homepage, not by this code.
+# The post index is generated by //users/tazjin/homepage, not by this
+# code.
 { depot, lib, ... }:
 
 let