feat: Add check for content

The `source/screens` directory is checked for any present file. This changes the permalink path for demo/index.liquid.

- Add fs module to eleventy config
- Add function to check for files
- Add demoPath to global data
- Change permalink for demo index to use demoPath

source/screens/demo/index.liquid

@@ -1,5 +1,5 @@
 ---
-permalink: "/"
+permalink: "{{ hippie.demoPath }}"
 title: Index
 ---
 {% assign pageId = page.fileSlug -%}