lib: load bash completion functions automatically

Fixes #8614

lib/completion.zsh

@@ -71,3 +71,6 @@ if [[ $COMPLETION_WAITING_DOTS = true ]]; then
   zle -N expand-or-complete-with-dots
   bindkey "^I" expand-or-complete-with-dots
 fi
+
+# automatically load bash completion functions
+autoload -Uz bashcompinit && bashcompinit