[split] Move export plugin to oh-my-fish/plugin-export

https://github.com/oh-my-fish/plugin-export

plugins/export/export.load

-function export
-  set var1 (echo $argv | cut -f1 -d=)
-  set var2 (echo $argv | cut -f2 -d=)
-  set -x -g $var1 $var2
-end