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
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| .. | ||
| src | ||
| Cargo.toml | ||