fix: migrate to ES6 module syntax in eleventy configuration

[nsunami]

This PR fixes the warning resulting from the project being in CommonJS. Migrated to ES6.

(node:77605) [MODULE_TYPELESS_PACKAGE_JSON] Warning: Module type of file:///Users/nami/dev/nami-writes/content/blog/blog.11tydata.js is not specified and it doesn't parse as CommonJS.
Reparsing as ES module because module syntax was detected. This incurs a performance overhead.
To eliminate this warning, add "type": "module" to /Users/nami/dev/nami-writes/package.json.
(Use `node --trace-warnings ...` to show where the warning was created)

[github-actions]

Preview deployment available at https://pr57-blog.namisunami.com