Copy static files

Would be a useful feature if you're building a browser extension, you may want to copy `manifest.json` and other static files to the `dist` folder.

Start by tracing tsup's build output flow and how files reach the dist folder; the issue specifically names manifest.json and other static files. Confirm the expected browser-extension use case and define done as copying those static files into dist during a build.

Written by the indexing model from the issue text.