Special characters in file names break `js` backend

The file name just gets inserted into js as a identifier names, so `./examples/llvm/hello-world.effekt`
generates the following js code:
```javascript
const $effekt = require("./effekt.js");

const $hello-world = { };

function main() {
const tmp344 = $effekt.println("hello world");

return $effekt.pure(tmp344);
}

module.exports = Object.assign($hello-world, { main: main });
```

Which is not correct js, as `-` can't be part of an identifier.