Fix check flexbox-gap

[ZhukDmitryOlegovich]

Hello,

I'm new to doing PR in open source, so if I'm doing something wrong, I'll be glad to receive feedback

I decided to add doiuse to my project and there are a couple of things that stop me from using it, so I’m going to make a series of PRs in which I propose corrections or improvements that will help me

---

1. Added a test case that doesn’t work for me
2. Corrected the flexbox-gap check function

[ZhukDmitryOlegovich]

Currently the css below is not defined as flexbox-gap.

.test {display:flex;gap:1rem;}
.test {
     display: /* some-comment */ flex;
     gap: 1rem;
}

I added these tests and corrected the function - now they are detected.

[clshortfuse]

I added some minor requests, but the logic looks good. Thanks for the PR!