rollup file.type is missing for the CSS output

Since `rollup@1.21.0` there is a requirement to set output `file.type` to be either 'chunk' or 'asset'. Currently, the `rollup-plugin-postcss` creates a CSS asset without type. The `rollup` mistakenly assumes that type is 'chunk', and classifies the build as a multi-chunk output build, expecting for `output.dir`. When it finds `output.file`, the build is failed with the following error:

```
Error: When building multiple chunks, the "output.dir" option must be used, not "output.file". To inline dynamic imports, set the "inlineDynamicImports" option.
```