Bump css-tree from 1.0.0-alpha.29 to 1.0.0-alpha.39

Bumps [css-tree](https://github.com/csstree/csstree) from 1.0.0-alpha.29 to 1.0.0-alpha.39.
- [Release notes](https://github.com/csstree/csstree/releases)
- [Changelog](https://github.com/csstree/csstree/blob/master/CHANGELOG.md)
- [Commits](csstree/csstree@v1.0.0-alpha.29...v1.0.0-alpha.39)

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

Author: dependabot-preview[bot]

package.json

@@ -26,7 +26,7 @@
   },
   "homepage": "https://github.com/eddiemoore/hydrogencss#readme",
   "devDependencies": {
-    "css-tree": "1.0.0-alpha.29",
+    "css-tree": "1.0.0-alpha.39",
     "rimraf": "2.6.2",
     "standard": "12.0.1"
   },