Add custom component for markdown with new-tab links

That way instead of having to do a Typescript cast every time we want to
render Markdown that opens links in a new tab, we can do the cast once
and just call a different component. We've done this already for the
lexbox frontend code in src/, so we'll do the same thing for the viewer.

frontend/viewer/src/lib/about/AboutDialog.svelte

@@ -2,9 +2,8 @@
   import { mdiInformationVariantCircle } from '@mdi/js';
   import { Button, Dialog, Toggle } from 'svelte-ux';
   import type { Readable } from 'svelte/store';
-  import Markdown from 'svelte-exmarkdown';
-  import NewTabLinkRenderer from './NewTabLinkRenderer.svelte';
-  import { onMount, type Component } from 'svelte';
+  import NewTabLinkMarkdown from '../markdown/NewTabLinKMarkdown.svelte';
+  import { onMount } from 'svelte';
 
   export let text: Readable<string>;
 
@@ -26,7 +25,7 @@
   </Button>
   <Dialog {open} on:close={toggleOff} class="w-[700px]">
     <div class="m-6 prose">
-      <Markdown md={$text} plugins={[{ renderer: { a: NewTabLinkRenderer as unknown as Component } }]} />
+      <NewTabLinkMarkdown md={$text} />
     </div>
     <div class="flex-grow"></div>
     <div slot="actions">

frontend/viewer/src/lib/markdown/NewTabLinKMarkdown.svelte

@@ -0,0 +1,10 @@
+<script lang="ts">
+  import Markdown, {type ComponentsMap} from 'svelte-exmarkdown';
+  import type {Component} from 'svelte';
+  import NewTabLinkRenderer from './NewTabLinkRenderer.svelte';
+
+  export let md: string;
+  const renderer: ComponentsMap = { a: NewTabLinkRenderer as unknown as Component };
+</script>
+
+<Markdown {md} plugins={[{ renderer: renderer }]} />