[BUG] trailing comma is not allowed on assert statements

**Describe the bug**
Title. An assert should behave just like a function call.

```cpp
main: () = {
    foo(a, b,); // works
    assert(true, "some_potentially_long_string",); // doesn't work
}
```
Output:
```
test.cpp2...
test.cpp2(3,48): error: expected ')' at the end of the contract (at ',')
```