Upgrade is-url-superb from ^3.0.0 to ^4.0.0

[davilima6]

WIP: tests do not pass. Is there anyone more familiar around? :)

  -         isUrl: false,
  +         isUrl: true,
            isVariable: false,
  -         parent: [Circular],
  +         parent: [Circular],

The major 4.0.0 drop dependency on url-regex, which has a up until now unpatched high vulnerability https://www.npmjs.com/advisories/1550

Next I remove the non-direct dependency of postcss-values-parser on url-regex, which I suppose was there to keep the version stable in case is-url-superb@^3.0.0 updated it, since it not imported anywhere in this postcss-values-parser.

This PR contains:

• bugfix
• feature
• refactor
• tests
• documentation
• metadata

Breaking Changes?

• yes
• no

New major of is-url-superb:

• requires Node.js 10
• no longer accepts protocol-relative URLs.

See https://github.com/sindresorhus/is-url-superb/releases/tag/v4.0.0

Please Describe Your Changes

This relates to #120

[shellscape]

Yeah for more context see the discussion starting here: #119 (comment)

[shellscape]

Closing in favor of #125. As noted on your other PR, I do sincerely and truthfully appreciate the work you put in on this.