fix: Use more idiomatic fish (#1042)

The use of `.` is deprecated in fish, unlike POSIX shells. We can avoid
calling `psub` and pipe directly into source which does the same thing.

https://fishshell.com/docs/current/cmds/source.html

lib/asdf.fish

@@ -7,7 +7,7 @@ function asdf
     switch "$command"
         case "shell"
             # source commands that need to export variables
-            . (asdf export-shell-version fish $argv | psub)
+            command asdf export-shell-version fish $argv | source # asdf_allow: source
         case '*'
             # forward other commands to asdf script
             command asdf "$command" $argv

test/banned_commands.bats

@@ -26,6 +26,7 @@ banned_commands=(
     "sort.*-V"
     "sort.*--sort-versions"
     # source isn't POSIX compliant. . behaves the same and is POSIX compliant
+    # Except in fish, where . is deprecated, and will be removed in the future.
     source
 )