Adding override keyword to the grammar

[jthoene]

With version 0.6 the realization of unimplemented functions must be declared with the keyword 'override'. This needs to be added in our grammar.

Example:

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.6.0;

abstract contract Feline {
    function utterance() public virtual returns (bytes32);
}

contract Cat is Feline {
    function utterance() public override returns (bytes32) { return "miaow"; }
}