Update deepmerge requirement from ^3.2.0 to ^4.0.0 (#898)

Author: dependabot-preview[bot]

package.json

@@ -41,7 +41,7 @@
     "codecov": "^3.3.0",
     "convert-hrtime": "^3.0.0",
     "dedent": "^0.7.0",
-    "deepmerge": "^3.2.0",
+    "deepmerge": "^4.0.0",
     "dezalgo": "^1.0.3",
     "js-yaml": "^3.13.1",
     "license-checker": "^25.0.1",