nushell: allow arbitrary environment variables

tests/modules/programs/nushell/env-expected.nu

@@ -1,4 +1,17 @@
 $env.FOO = 'BAR'
 
 
-$env.BAR = $'(echo BAZ)'
+load-env {
+    "ENV_CONVERSIONS": {
+        "PATH": {
+            "from_string": ({|s| $s | split row (char esep) })
+            "to_string": ({|v| $v | str join (char esep) })
+        }
+    }
+    "FOO": "BAR"
+    "LIST_VALUE": [
+        "foo"
+        "bar"
+    ]
+    "PROMPT_COMMAND": ({|| "> "})
+}