Add syntax highlighting to patch and commit changesets

- This commit doesn't change the way syntax highlighting is done.
- But due to changes in `radicle-httpd` we can highlight the entire
  files of a changeset, and then replace the plain text lines with
  highlighted ones.
- Also highlights only the files currently visible of a changeset.

Signed-off-by: Sebastian Martinez <me@sebastinez.dev>

src/components/Observer.svelte

@@ -0,0 +1,41 @@
+<script lang="ts" context="module">
+  export function intersection(
+    node: HTMLElement,
+    observer: IntersectionObserver | undefined,
+  ) {
+    if (!observer) return;
+    observer.observe(node);
+    return {
+      destroy() {
+        observer.unobserve(node);
+      },
+    };
+  }
+</script>
+
+<script lang="ts">
+  import { onDestroy } from "svelte";
+
+  let observer: IntersectionObserver | undefined = undefined;
+  let filesVisibility = new Set<string>();
+
+  if ("IntersectionObserver" in window) {
+    observer = new IntersectionObserver(entries => {
+      entries.forEach(entry => {
+        if (entry.isIntersecting) {
+          filesVisibility = filesVisibility.add(
+            entry.target.id.replace("observer:", ""),
+          );
+        }
+      });
+    });
+  }
+
+  onDestroy(() => {
+    if (observer) {
+      observer.disconnect();
+    }
+  });
+</script>
+
+<slot {observer} {filesVisibility} />