Add ’main‘ field to package.json (to make use of ender.js’ package management capabilities)

For tablesort to work with ender.js, you have to separately link the tablesort.js file. We could benefit from ender’s package management by adding the `main` field to package.json:

```js
{
  // package.json definitions
  "ender": "./ender.js",
  "main": "./src/tablesort.js",
  // …
}
```

The following ender call would then automatically append the tablesort source code to the generated ender.js file:
```bash
ender add tablesort
```

This works well for me. But since I am not very experienced with package.json definitions, I’m not sure if this may have any consequences for other npm packages.