feat: Add Valtown embed support

shared/editor/embeds/Valtown.tsx

@@ -0,0 +1,19 @@
+import * as React from "react";
+import Frame from "../components/Frame";
+import { EmbedProps as Props } from ".";
+
+export default function Valtown(props: Props) {
+  const { matches } = props.attrs;
+  const valId = matches[1];
+
+  return (
+    <Frame
+      {...props}
+      src={`https://www.val.town/embed/${valId}`}
+      title="Valtown"
+      border
+    />
+  );
+}
+
+Valtown.ENABLED = [/^https?:\/\/(?:www.)?val\.town\/(?:v|embed)\/(.*)$/];