Implement CSharpScript::is_valid()

modules/mono/csharp_script.cpp

@@ -2594,10 +2594,6 @@ void CSharpScript::update_exports() {
 #endif
 }
 
-bool CSharpScript::is_valid() const {
-	return true; //TODO return false if invalid
-}
-
 bool CSharpScript::has_script_signal(const StringName &p_signal) const {
 	if (_signals.has(p_signal))
 		return true;

modules/mono/csharp_script.h

@@ -169,7 +169,7 @@ public:
 	virtual void update_exports();
 
 	virtual bool is_tool() const { return tool; }
-	virtual bool is_valid() const;
+	virtual bool is_valid() const { return valid; }
 
 	virtual Ref<Script> get_base_script() const;
 	virtual ScriptLanguage *get_language() const;