add rule to warn about properties that are overridden by object spread

[ajafff]

let foo = {
    x: 1, // always overridden
    ... {x: 2}
};

let bar = Boolean() ? {prop: 1} : {};
let baz = {
    y: 0,
    prop: 0, // not always overridden, because the property is optional in 'bar'
    ...bar,
};

// this could be an enhancement:
let bas = {
    ...foo, // will be overridden completely by 'x: 3'
    ...bar, // will be overridden completely by '...baz'
    x: 3,
    ...baz, // NOT completely overridden by '...bar', because it doesn't contain 'y'
    ...bar,
    // uncommenting the following line results in an error on '...baz'
    // y: 1,
};

Ref: microsoft/TypeScript#23340