External formatter don't output stderr

**Is your feature request related to a problem? Please describe.**
Trying to use sqlfmt as an external formatter but it outputs some info on stderr and can't get rid of that from the output e.g. 2>nul doesn't work in that field.

install sqlfmt: pip install shandy-sqlfmt[jinjafmt]

use with sqlfmt - in DBeaver as External formatter

outputs formatted query and also:
```
1 file formatted.
0 files left unchanged.
- formatted.
```

**Describe the solution you'd like**
Not outputting stderr from external formatter

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
Add any other context or screenshots about the feature request here.