Update dependency core-js to v2.6.12

Author: renovate[bot]

yarn.lock

@@ -757,8 +757,7 @@ convert-source-map@^1.1.0:
   resolved "https://registry.yarnpkg.com/convert-source-map/-/convert-source-map-1.3.0.tgz#e9f3e9c6e2728efc2676696a70eb382f73106a67"
 
 core-js@^2.2.2, core-js@^2.4.0:
-  version "2.4.1"
-  resolved "https://registry.yarnpkg.com/core-js/-/core-js-2.4.1.tgz#4de911e667b0eae9124e34254b53aea6fc618d3e"
+  version "2.6.12"
 
 core-util-is@~1.0.0:
   version "1.0.2"