Add ability to remove .html extension from path

[azat-io]

Hello! Thank you for awesome Astro plugin!

I'm using Netlify with Pretty URLs option.

I remove .html ext from URLs: /about.html -> /about.

When I use astro-shield package, this plugin creates _headers file with full URL:

/about.html
  content-security-policy: ...

For this reason, the CSP does not work on the page. Can we add the removeHtmlExtension option so that the plugin can be used with Pretty URLs?