support injecting single static files into nitro public folder

[danielroe]

By default, we could add favicon.ico (1x1 transparent gif) and empty robots.txt if there's no explicit public/ folder. Modules might also want to inject other files (like sw.js).

In order to do this we need support within nitro for injecting single static files into the output.

addAsset('favicon.ico', ...)
addBuildAsset('icon.hash123.png')

See https://github.com/nuxt/framework/discussions/2294#discussioncomment-1756029.

[pi0]

With nitro, we can add multiple publicAssets dirs with their prefix and directory.

Supporting individual and virtual public assets could be also a nice idea but I need more reasons to include it (feel free to open a discussion in nitro repo)

Also we have unjs/serve-placeholder that automatically handles cases such as favicon.ico, robots.txt and *.js (not found) (to be enabled for production as well via nitrojs/nitro#94)