feat: split loadenv from readenv to load any file

cli/fish/config.fish

@@ -70,8 +70,7 @@ if status is-interactive
     # === auto cd into last activated directory
     test "$PWD" = "$HOME" && cd $last_pwd
 
-    function readenv --on-variable PWD
-        if test -r .env
+    function loadenv
         while read -l line
             set -l line (string trim $line)
             if [ -z "$line" ]
@@ -82,7 +81,12 @@ if status is-interactive
             end
             set -l kv (string split -m 1 = -- $line)
             set -gx $kv
-            end < .env
+        end < $argv[1]
+    end
+
+    function readenv --on-variable PWD
+        if test -r .env
+            loadenv .env
        end
     end
 end