Add custom highlighter for rehype-pretty-code

To make rehype-pretty-code work properly, shiki files need to be
included in production environment on Vercel. By reading shiki files,
Vercel becomes aware that those files should be included.

vercel/next.js#52711 (comment)

src/lib/mdToHtml.ts

@@ -1,4 +1,5 @@
 import rehypeMermaid from "./rehypeMermaid";
+import getHighlighter from "@/lib/shikiHilighter"
 
 import { remark } from "remark";
 import remarkGfm from "remark-gfm";
@@ -28,7 +29,7 @@ const mdToHtml: (md: string) => Promise<string> = async (md) => {
     .use(rehypeSlug)
     .use(rehypeAutolinkHeadings, { behavior: "wrap" })
     .use(rehypeMermaid)
-    .use(rehypePrettyCode)
+    .use(rehypePrettyCode, { getHighlighter })
     .use(rehypeStringify)
     .process(md);
   return result.toString();