Add _binary_exists and use in github-cli completion

gh was also used as an git alias, and caused _command_exists to not fail as expected

completion/available/github-cli.completion.bash

@@ -2,7 +2,7 @@
 cite "about-completion"
 about-completion "GitHub CLI completion"
 
-if _command_exists gh; then
+if _binary_exists gh; then
   if _command_exists brew; then
     _log_warning "You don't need github-cli completion enabled if you have system completion enabled"
   fi