App::_process: add conditional logic for old vs. new help processing

Some use cases require altering subcommand/option states (and thus the help
output) based on a flag stored in the config file. In this case, running
`_process_callbacks()` before `_process_help_flags()` is necessary.

A new compile-time option `CLI11_USE_OLD_HELP_PROCESSING` restores this
“old” behavior by executing callbacks prior to help flag processing.

Extends CLIUtils#1186

Author: VolkerChristian

include/CLI/impl/App_inl.hpp

@@ -1413,9 +1413,11 @@ CLI11_INLINE void App::_process_requirements() {
 }
 
 CLI11_INLINE void App::_process() {
+    #ifndef CLI11_USE_OLD_HELP_PROCESSING
     // help takes precedence over other potential errors and config and environment shouldn't be processed if help
     // throws
     _process_help_flags();
+    #endif // CLI11_USE_OLD_HELP_PROCESSING
     std::exception_ptr config_exception;
     try {
         // the config file might generate a FileError but that should not be processed until later in the process
@@ -1427,12 +1429,20 @@ CLI11_INLINE void App::_process() {
     } catch(const CLI::FileError &) {
         config_exception = std::current_exception();
     }
+    #ifndef CLI11_USE_OLD_HELP_PROCESSING
     // callbacks and requirements processing can generate exceptions which should take priority
     // over the config file error if one exists.
     _process_requirements();
+    #endif // CLI11_USE_OLD_HELP_PROCESSING
 
     _process_callbacks();
 
+    #ifdef CLI11_USE_OLD_HELP_PROCESSING
+    _process_help_flags();
+
+    _process_requirements();
+    #endif // CLI11_USE_OLD_HELP_PROCESSING
+
     if(config_exception) {
         std::rethrow_exception(config_exception);
     }