[BUG] trailing comma is not allowed on assert statements

[Dylam De La Torre]

Describe the bug
Title. An assert should behave just like a function call.

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 ',')

[Herb Sutter]

Thanks! Fixed by #1121.