Add extra types.

source/env.d.ts

@@ -0,0 +1,12 @@
+// eslint-disable-next-line @typescript-eslint/triple-slash-reference
+/// <reference types="astro/client" />
+
+import {type JSX} from "preact";
+
+declare global {
+  /** Shorthand for {@linkcode JSX.Element}. */
+  type JsxElement = JSX.Element;
+
+  /** An optional {@linkcode JsxElement}. */
+  type OptionalJsxElement = JsxElement | undefined;
+}