Move away from deprecated shiki highlighting function (#6016)

shikijs/shiki#702 has more details about the
upstream deprecation and change.

src/_11ty/plugins/highlight.js

@@ -1,4 +1,4 @@
-import { getHighlighter } from 'shiki';
+import { getSingletonHighlighter } from 'shiki';
 import dashLightTheme from '../syntax/dash-light.js';
 
 /**
@@ -17,7 +17,7 @@ import dashLightTheme from '../syntax/dash-light.js';
  *   configure syntax highlighting for.
  */
 export async function configureHighlighting(markdown) {
-  const highlighter = await getHighlighter({
+  const highlighter = await getSingletonHighlighter({
     langs: [
       'dart',
       'yaml',
@@ -38,10 +38,9 @@ export async function configureHighlighting(markdown) {
       'cmd',
       'plaintext',
     ],
+    themes: [dashLightTheme],
   });
 
-  await highlighter.loadTheme(dashLightTheme);
-
   markdown.renderer.rules.fence = function (tokens, index) {
     const token = tokens[index];