style(bash-v2): use herestring in activehelp extraction

Herestrings read cleaner than process substitutions, and work in posix
mode (but we do and will have some process substitutions so this doesn't
matter much). Both approaches may end up using temporary files.

bash_completionsV2.go

@@ -183,7 +183,7 @@ __%[1]s_extract_activeHelp() {
             # Not an activeHelp line but a normal completion
             completions+=("$comp")
         fi
-    done < <(printf "%%s\n" "${out}")
+    done <<<"${out}"
 }
 
 __%[1]s_handle_completion_types() {