feat: upgrade fetch-mock from 11.1.5 to 12.0.0

Snyk has created this PR to upgrade fetch-mock from 11.1.5 to 12.0.0.

See this package in npm:
fetch-mock

See this project in Snyk:
https://app.snyk.io/org/contentstack-devex/project/ade506e8-b348-4d8d-a8ce-8dfcb6f9fc39?utm_source=github&utm_medium=referral&page=upgrade-pr

Author: snyk-bot

package-lock.json

@@ -102,7 +102,7 @@
     "@contentstack/utils": "^1.3.12",
     "cheerio": "^1.0.0",
     "es6-promise": "^4.2.8",
-    "fetch-mock": "^11.1.5",
+    "fetch-mock": "^12.0.0",
     "isomorphic-fetch": "^3.0.0",
     "localStorage": "1.0.4",
     "qs": "^6.13.0"