feat(furc/parser): add pre- and post- conditions

furc/src/main.cpp

@@ -6,7 +6,7 @@ int main(void) {
     furlang::arena arena;
 
     std::string_view content = R"(
-        func main(argc: u64) -> s32 {
+        func main(argc: u64) -> s32 pre(arc > 1) {
             x: s32 = 1 + 2 * 3;
             if (x == 9) return 1;
             else return 0;