Add .js extension to import statement for node16

For node16 / nodenext support, this should append .js to the import statement.

So:
```
  tsTypes: {} as import("./index.typegen").Typegen0,
```

should become:

```
  tsTypes: {} as import("./index.typegen.js").Typegen0,
```

I believe this would be backwards compatible on non-node16 as well, so should be safe to change.