main: Declare variable local to fix WARN_CREATE_GLOBAL error

highlighters/main/test-data/opt-shwordsplit1.zsh

@@ -29,7 +29,7 @@
 # -------------------------------------------------------------------------------------------------
 
 setopt shwordsplit
-EDITOR='ed -s'
+local EDITOR='ed -s'
 
 ed() { command ed "$@" }