Add ability to set `ENVIRONMENT VARIABLES` from the
config and the layout files.
example:
```
env:
ZELLIJ_CONFIG:
DEFAULT
```
or
```
env:
ZELLIJ_LAYOUT_NAME:
BUILD_SESSION
```
If two keys conflict (configuration and layout), then the key
from the layout is used.
fixes: #1059
|
||
|---|---|---|
| .. | ||
| unit | ||
| command_is_executing.rs | ||
| input_handler.rs | ||
| lib.rs | ||
| os_input_output.rs | ||
| stdin_handler.rs | ||