Support CSP headers generation for static content on Vercel

[castarco]

To improve the usability of Astro-Shield, it would be good to support the generation of headers metadata specific for the Vercel provider, so we can generate CSP headers for static content.

Reference

• See https://vercel.com/docs/projects/project-configuration#headers

Status

• [X] Main implementation (#111, #121)
• [ ] Support for "dev resources" (dev toolbar loaded in deployed development branches)

[castarco]

Accidentally closed