Tier1: add function for checking if SDK ConVar's are registered

src/public/tier1/convar.h

@@ -345,4 +345,6 @@ FORCEINLINE const char* ConVar::GetString(void) const
 void ConVar_Register(int nCVarFlag = 0, IConCommandBaseAccessor* pAccessor = NULL);
 void ConVar_Unregister();
 
+bool ConVar_IsRegistered();
+
 #endif // CONVAR_H

src/tier1/convar.cpp

@@ -76,6 +76,11 @@ void ConVar_Unregister()
 	s_bRegistered = false;
 }
 
+bool ConVar_IsRegistered()
+{
+	return s_bRegistered;
+}
+
 ConCommandBase::~ConCommandBase(void)
 {
 	if (m_pszCustomUsageString)