feature: basic security headers

[jbmoelker]

Changes

• Adds basic security headers using Cloudflare Pages _headers configuration file.

before:

after:

Associated issue

Resolves #56

How to test

1. Open Network tab in DevTools
2. Open preview link
3. Verify responses have headers configured in this PR
4. Check and compare tests under changes section above
5. Verify security report improved from D to B.

Checklist

• [x] I have performed a self-review of my own code
• [x] I have made sure that my PR is easy to review (not too big, includes comments)
• I have added/updated tests to prove that my feature works (if not possible please explain why)
• I have made changes to the README and if the change affects the project setup (npm commands changed, new service added, environmental variable added)
• I have added a decision log entry if the change affects the architecture or changes a significant technology
• [x] I have notified a reviewer

[cloudflare-pages[bot]]

Deploying with    Cloudflare Pages

Latest commit:	00f71e5
Status:	✅  Deploy successful!
Preview URL:	https://bd6347d6.head-start.pages.dev
Branch Preview URL:	https://feat-security-headers.head-start.pages.dev

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