fix: support Single Executable Applications

[sebamarynissen]

I'm using this module with Node's new Single Executable Application functionality. Currently this requires you to bundle your application into a single JavaScript file, which I'm doing with esbuild. However, when importing the module, it chokes over

const __dirname = path.dirname(fileURLToPath(import.meta.url));

because after bundling with esbuild, import.meta.url is undefined.

This PR fixes that by falling back to process.execPath in case import.meta.url is undefined. This also means that if someone wants to use the bundled xdg binary, they can distribute it alongside the sea binary and it should work.