[CSM] sound_play & sound_stop support + client_lua_api doc (#5096)

* squashed: CSM: Implement register_globalstep
  * Re-use fatal error mechanism from server to disconnect client on CSM error
  * Little client functions cleanups

* squashed: CSM: add core.after function
  * core.after is shared code between client & server
  * ModApiUtil get_us_time feature enabled for client

src/script/clientscripting.cpp

@@ -22,6 +22,7 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 #include "client.h"
 #include "cpp_api/s_internal.h"
 #include "lua_api/l_client.h"
+#include "lua_api/l_sound.h"
 #include "lua_api/l_util.h"
 
 ClientScripting::ClientScripting(Client *client):
@@ -51,4 +52,5 @@ void ClientScripting::InitializeModApi(lua_State *L, int top)
 {
 	ModApiUtil::InitializeClient(L, top);
 	ModApiClient::Initialize(L, top);
+	ModApiSound::Initialize(L, top);
 }