ignore yarn warnings when upgrading global packages

Until yarnpkg/yarn#6672 is resolved,
this kind of hacky solutions is the only way not to go bananas with
yarn's unactionable-for-me-warning-messages.

fish/functions/up.fish

@@ -20,7 +20,7 @@ function up
     pip-sync ~/personal/dotfiles/pip/requirements.txt
 
     echo -e '####################################\n# Yarn \n####################################'
-    yarn global upgrade --silent
+    yarn global upgrade --silent 2>&1 | ack --invert-match warning
 
     echo -e '\n####################################\n# Oh-My-Fish \n####################################'
     omf update;