Consider adding a "custom entry point" flag for running utility scripts

One annoyance today is that e.g. my TIL site uses utility scripts, but they only run on POSIX. If I could convert those to Lua and I had a Lua script runner, I could actually run those utility scripts on any supported OS (POSIX or not).

Obviously, this would mainly just be to support Windows, since it's trivial to run everything on Linux/BSD and I don't have any macOS computers.