Support Node.js v25

[yamcodes]

Node.js v25 just came out, but as seen in the failing workflows of #230, we cannot support it out of the box given a post-install issue with fumadocs-mdx, as documented fuma-nama/fumadocs#2456.

This is important since we want to provide reasonable support for arkenv and @arkenv/vite-plugin (and any future library we publish in this project), and I would deem support for the latest Node.js as "reasonable".

There are a few pathways to support this change:

1. Fix the post-install issue upstream (in fuma-nama/fumadocs or nodejs/node), this should make everything work in Node.js v25.
2. In case (1) is not approved for some reason (or until it does), use a fork. This might require publishing an npm package or consume the package directly from GitHub.
3. Fix the post-install issue using a patch. This is a bandaid solution.
4. Complicate our workflows such that in case of Node.js 25, www is omitted from the workflow. This might involve the usage of pnpm filtering, Turborepo filtering, or even more complicated conditional logic. This solution is sound since we don't really need to use Node.js 25 for www, our infra supports LTS just fine.
5. Use a custom post-install script.
6. Migrate away from Fumadocs. Candidates include Nextra and Mintlify. This is obviously the most expensive solution.