feat: add get variable

Signed-off-by: kjuulh <contact@kjuulh.io>

churn-tasks/wit/world.wit

@@ -4,6 +4,7 @@ interface process {
     resource process {
         constructor();
         run-process: func(inputs: list<string>) -> string;
+        get-variable: func(key: string) -> string;
     }
 }