Toward portability

[david-crespo]

Lots of little bits progress toward #232. The main things are

• All process.argv stuff happens at the very top level, with relative path destination dir arg resolved to absolute paths immediately (relative to cwd) and passed down the call chain
• IO init happens inside generator functions instead of at module top level
• Static files we copy over are resolved relative to the script's location rather than CWD, which means we can call the script directly from anywhere without having to get weird with npm run --prefix
• Use tsx instead of esbuild-register to run the thing, mostly because I ran into an error when trying to use import.meta.url and didn't feel like figuring it out with a tool I don't even like