Bump broccoli-merge-trees from 3.0.2 to 4.2.0 (#494)

Bumps [broccoli-merge-trees](https://github.com/broccolijs/broccoli-merge-trees) from 3.0.2 to 4.2.0.
- [Release notes](https://github.com/broccolijs/broccoli-merge-trees/releases)
- [Changelog](https://github.com/broccolijs/broccoli-merge-trees/blob/master/CHANGELOG.md)
- [Commits](broccolijs/broccoli-merge-trees@v3.0.2...v4.2.0)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

package.json

@@ -38,7 +38,7 @@
     "broccoli-debug": "^0.6.5",
     "broccoli-filter": "^1.3.0",
     "broccoli-funnel": "^3.0.3",
-    "broccoli-merge-trees": "^3.0.2",
+    "broccoli-merge-trees": "^4.2.0",
     "broccoli-persistent-filter": "^2.3.1",
     "broccoli-plugin": "1.3.1 - 3",
     "broccoli-source": "^3.0.0",