Add support for asset directories

[alexchandel]

Often one needs to include a directory of static assets.

fossilize -a assets fails for a directory:

Platforms: win-x64
Cleaning up dist-bin...
Bundling src/server.js...
Resolved Node.js version 'local' to 22.17.1

  dist-bin\bin\server.exe.cjs  1011.2kb

⚡ Done in 163ms
Failed to `run .node-cache\node-v22.17.1-win-x64.exe --experimental-sea-config dist-bin\sea-config.json`

Cannot read asset C:\src\server\assets: illegal operation on a directory

[BYK]

Thanks for reporting this! I'll see if I can implement this easily.