Remove event dispatcher and make required props explicit

Svelte has deprecated the event dispatcher functionality and instead
works with callback functions that are called when an event should in
the past have been called.

Also `export let var: string` is in svelte something that can't be
optional, so we should type it out as `export let var = string |
undefined = undefined` or make sure we always pass that prop.

src/components/Header.svelte

@@ -6,7 +6,7 @@
   import Popover from "./Popover.svelte";
   import ThemeSwitch from "./ThemeSwitch.svelte";
 
-  export let currentPage: string;
+  export let currentPage: string | undefined = undefined;
 </script>
 
 <style>

src/components/Link.svelte

@@ -1,15 +1,11 @@
-<script lang="ts" strictEvents>
+<script lang="ts">
   import type { Route } from "@app/lib/router/definitions";
 
-  import { createEventDispatcher } from "svelte";
   import { push, routeToPath } from "@app/lib/router";
 
   export let route: Route;
   export let disabled: boolean = false;
-
-  const dispatch = createEventDispatcher<{
-    afterNavigate: null;
-  }>();
+  export let afterNavigate: (() => void) | undefined = undefined;
 
   function navigateToRoute(event: MouseEvent): void {
     event.preventDefault();
@@ -18,7 +14,9 @@
     }
 
     void push(route);
-    dispatch("afterNavigate");
+    if (afterNavigate) {
+      afterNavigate();
+    }
   }
 </script>