Update unified requirement to ^7.0.0 (#10474)

Updates the requirements on [unified](https://github.com/unifiedjs/unified) to permit the latest version.
- [Release notes](https://github.com/unifiedjs/unified/releases)
- [Commits](https://github.com/unifiedjs/unified/commits/7.0.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

package.json

@@ -162,7 +162,7 @@
     "stylelint": "9.2.0",
     "stylelint-config-standard": "^18.0.0",
     "typescript": "~2.8.1",
-    "unified": "^6.1.5",
+    "unified": "^7.0.0",
     "values.js": "^1.0.3",
     "xhr2": "^0.1.3"
   },