Fix multi-line string highlighting with single quotes.

modules/gdscript/gdscript_editor.cpp

@@ -54,6 +54,7 @@ void GDScriptLanguage::get_string_delimiters(List<String> *p_delimiters) const {
 	p_delimiters->push_back("\" \"");
 	p_delimiters->push_back("' '");
 	p_delimiters->push_back("\"\"\" \"\"\"");
+	p_delimiters->push_back("''' '''");
 }
 
 bool GDScriptLanguage::is_using_templates() {