Don't run pp-flags on Reason files

**Example**: https://github.com/IwanKaramazow/pp-demo-1
**bs-plaform version**: 5.2.1 and above
**steps to reproduce**
```
git clone https://github.com/IwanKaramazow/pp-demo-1
npm install
nom run build
``` 
`pp-flags` from `bsconfig.json` is run on Reason files. This wasn't the case in bsb 4.
Pp-flags shouldn't be run on Reason files as refmt outputs a binary format.

