Throw a better error when the `output` folder in Eleventy matches Vite’s `publicDir` option

[zachleat]

publicDir: "public" is a reserved Vite directory and Eleventy can’t use the same value for its output directory. We need a better error message here.

Found via @aarongoldenthal on this issue: https://github.com/11ty/eleventy-plugin-vite/issues/22#issuecomment-1407158803

Workaround is to change viteOptions.publicDir (via https://github.com/11ty/eleventy-plugin-vite#options) or to change Eleventy’s output directory.