move `LESS` export from zshenv to a custom file

oh-my-zsh now writes `LESS` in `lib/misc.zsh`, and this is invoked after zshenv.

zsh/omz-custom/less.zsh

@@ -0,0 +1 @@
+export LESS='-eirMX'

zsh/zshenv

@@ -8,7 +8,6 @@
 #
 
 ## don't close less when I am done reading
-export LESS='-eirMX'
 export EDITOR='vim'
 
 ## paths