build: 🔧 use manual feed template instead of virtual one

In order to display the title image in the feed.xml like in the posts on the website, a more customizable variant is needed.

src/feed.njk

@@ -0,0 +1,40 @@
+---json
+{
+  "permalink": "feed.xml",
+  "eleventyExcludeFromCollections": true,
+  "metadata": {
+    "title": "Sebin's Blog",
+    "description": "Writing about stuff I have vague interests in and commenting on stuff I read.",
+    "language": "en",
+    "base": "https://blog.sebin-nyshkim.net/",
+    "author": {
+      "name": "Sebin Nyshkim"
+    }
+  }
+}
+---
+<?xml version="1.0" encoding="utf-8"?>
+<feed xmlns="http://www.w3.org/2005/Atom" xml:lang="{{ metadata.language or page.lang }}">
+  <title>{{ metadata.title }}</title>
+  <subtitle>{{ metadata.description }}</subtitle>
+  <link href="{{ permalink | htmlBaseUrl(metadata.base) }}" rel="self"/>
+  <link href="{{ metadata.base | addPathPrefixToFullUrl }}"/>
+  <updated>{{ collections.posts | getNewestCollectionItemDate | dateToRfc3339 }}</updated>
+  <id>{{ metadata.base | addPathPrefixToFullUrl }}</id>
+  <author>
+    <name>{{ metadata.author.name }}</name>
+  </author>
+  {%- for post in collections.posts | reverse %}
+  {%- set absolutePostUrl %}{{ post.url | htmlBaseUrl(metadata.base) }}{% endset %}
+  <entry>
+    <title>{{ post.data.title }}</title>
+    <link href="{{ absolutePostUrl }}"/>
+    <updated>{{ post.date | dateToRfc3339 }}</updated>
+    <id>{{ absolutePostUrl }}</id>
+    <content type="html">
+      &lt;img src="{{ post.data.image.src }}.webp?width=1200" alt="{{ post.data.image.alt }}"&gt;
+      {{ post.content | renderTransforms(post.data.page, metadata.base) }}
+    </content>
+  </entry>
+  {%- endfor %}
+</feed>