Fix false positives for vendor-prefixed image-set w/ unit "x" in unit-no-unknown

[srawlins]

Clearly describe the bug

The unit-no-unknown rule reports a false positive when using the "x" unit in a value for a vendor-prefixed image-set property value. E.g.

a { background-image: -webkit-image-set('img-1x.jpg' 1x, 'img-2x.jpg' 2x) }

caniuse mentions that -webkit-image-set is a valid vendor-prefixed version of image-set: https://caniuse.com/#feat=css-image-set

(slight change to "ignore x unit in image-set" test case in lib/rules/unit-no-unknown/tests/index.js)

Which rule, if any, is the bug related to?

unit-no-unknown

What code is needed to reproduce the bug?

a { background-image: -webkit-image-set('img-1x.jpg' 1x, 'img-2x.jpg' 2x) }

What stylelint configuration is needed to reproduce the bug?

{
  "rules": {
    "unit-no-unknown": "always"
  }
}

Which version of stylelint are you using?

13.2.1

How are you running stylelint: CLI, PostCSS plugin, Node.js API?

CLI; npm run test

Does the bug relate to non-standard syntax (e.g. SCSS, Less etc.)?

No

What did you expect to happen?

No warnings to be flagged.

What actually happened (e.g. what warnings or errors did you get)?

The following warnings were flagged:

{
  "column": 54,
  "line": 1,
  "rule": "unit-no-unknown",
  "severity": "error",
  "text": "Unexpected unknown unit \"x\" (unit-no-unknown)",
}

I can write and send a PR.