Support for child combinator

[bangseongbeom]

Current version supports only Descendant combinator(.sel1 .sel2).
Child combinator(.sel1 > .sel2) should be also supported.

Input:

.a {
    --color: red;
}

.a > .b {
    color: var(--color);
}

Output:

.a > .b {
    color: undefined;
}

Expectation:

.a > .b {
    color: red;
}

[MadLittleMods]

Thank you for another great bug report @bangseongbeom!

I have added full support for the child combinator and direct-descendants.

I also added three tests to ensure we do not regress.