Allows catching a chatcommand call just after the command and the
parameters are parsed but before its existence is checked and before the
corresponding function is run. Returning `true` from a callback function
will prevent default handling of the command leaving mods to handle the
command manually.
* [CSM] Add send_chat_message and run_server_chatcommand API functions
* Add client-side chat message rate limiting
* Limit out chat queue size
* [CSM] Add minetest.clear_out_chat_queue API function and .clear_chat_queue chatcommand
* Last fixes/cleanups before merge
* client side mods are located in clientmods/
* move builtin/preview.lua to clientmods/preview/init.lua as a preview mod
* refactor ModConfiguration class to work properly with client and server using child objects
* move some Server constructor mod load code to ModConfiguration to reduce code duplication between client and server
* remove mods.{cpp,h} unused functions
* use UNORDERED_SET instead of std::set in some modspec storages
