add Content Security Policy

[simpixelated]

Lots of good discussion on how to do it here: 11ty/eleventy#2435. Some options:

• create a Netlify _headers file, copy this to the build folder
• https://github.com/google/eleventy-high-performance-blog/blob/main/_11ty/apply-csp.js
• https://github.com/nhoizey/nicolas-hoizey.photo/blob/main/src/_headers.njk
• https://github.com/MarcelloTheArcane/netlify-plugin-csp-generator

References:

• https://content-security-policy.com/examples/netlify/

[simpixelated]

Should solve lighthouse error: https://github.com/simpixelated/simpixelated.com/actions/runs/4234328572/jobs/7356459606#step:4:153