variables as values?

Is it possible to set and use variables within commands?

This works (focuses on a term window titled 'qalc':

``` bash
alt + cmd - 0x29 : yabai -m window $(yabai -m query --windows | jq '.[] | select(.title == "qalc") | .id') --focus
```

But this gives the error "undeclared identifier":
```bash
QALC=$(yabai -m query --windows | jq '.[] | select(.title == "qalc") | .id')

alt + cmd - 0x29 : yabai -m window ${QALC} --focus
```

Start by reproducing the two command forms from the report and trace how skhd parses and executes command text; the issue names no source file or test. Done would mean establishing whether variables are supported in configured commands and documenting or testing the resulting behavior.

Written by the indexing model from the issue text.