feat(nix): support new nix shell command (#4724)

* Support `nix shell`

* Remove unnecessary `Debug` implementation

* Add test to detect false positive

* Improve detection of `/nix/store` in $PATH

* Add docs about unknown state

* Gate under `heuristic` flag

* Regenerate config schema

.github/config-schema.json

@@ -989,10 +989,12 @@
       "default": {
         "disabled": false,
         "format": "via [$symbol$state( \\($name\\))]($style) ",
+        "heuristic": false,
         "impure_msg": "impure",
         "pure_msg": "pure",
         "style": "bold blue",
-        "symbol": "❄️  "
+        "symbol": "❄️  ",
+        "unknown_msg": ""
       },
       "allOf": [
         {
@@ -4066,9 +4068,17 @@
           "default": "pure",
           "type": "string"
         },
+        "unknown_msg": {
+          "default": "",
+          "type": "string"
+        },
         "disabled": {
           "default": false,
           "type": "boolean"
+        },
+        "heuristic": {
+          "default": false,
+          "type": "boolean"
         }
       },
       "additionalProperties": false