Inherit vars from current environment

Perhaps a notation such as:

```
foo: inherit
bar
baz
```

in `.vars` would be a way to signify that you're using a specific var that's currently available in the shell environment, without retyping it. eg. folder directory. Not sure if it's worth the complication.

Thoughts? @bucaran

Start by inspecting how koi currently reads `.vars` and resolves variables from the shell environment. Clarify the proposed `inherit` syntax, its behavior when a variable is missing, and whether the added complexity is acceptable before defining what completion would require.

Written by the indexing model from the issue text.