Add support for top level file definitions as introduced in solc 0.8.22

[0xalpharush]

Describe the desired feature

event MyEvent(uint256 indexed a, uint256 indexed b);

contract Test {
    function test() public {
        emit MyEvent(1, 2);
    }
}

This requires adding a new top level parser to SlitherCompilationUnitSolc and updating find_top_level to include events