IIFE uses the same name format as CJS?

When exporting IIFE format, name of the output file is the same as for CJS format so CJS is overwritten

```
> microbundle --format cjs,iife --name mynth_test

Build "test" to dist:
567 B: test.cjs.gz
437 B: test.cjs.br
573 B: test.cjs.gz
442 B: test.cjs.br
```