babel: Specify core-js minor version.

This enables core-js modules for proposals marked as finished between
core-js 3.0 and 3.6.  It’s recommended upstream, although it has no
current effect for us.

The PROVISION_VERSION bump is skipped because there is no actual
dependency change.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

babel.config.js

@@ -5,7 +5,7 @@ module.exports = {
         [
             "@babel/preset-env",
             {
-                corejs: 3,
+                corejs: "3.6",
                 loose: true, // Loose mode for…of loops are 5× faster in Firefox
                 shippedProposals: true,
                 useBuiltIns: "usage",

package.json

@@ -21,7 +21,7 @@
     "cache-loader": "^4.0.0",
     "clean-css": "^4.2.1",
     "clipboard": "^2.0.4",
-    "core-js": "^3.0.1",
+    "core-js": "^3.6.5",
     "css-loader": "^4.3.0",
     "emoji-datasource-google": "^5.0.1",
     "emoji-datasource-google-blob": "npm:emoji-datasource-google@^3.0.0",

yarn.lock

@@ -3278,7 +3278,7 @@ core-js@^2.4.0:
   resolved "https://registry.yarnpkg.com/core-js/-/core-js-2.6.11.tgz#38831469f9922bded8ee21c9dc46985e0399308c"
   integrity sha512-5wjnpaT/3dV+XB4borEsnAYQchn00XSgTAWKDkEqv+K8KevjbzmofK6hfJ9TZIlpj2N0xQpazy7PiRQiWHqzWg==
 
-core-js@^3.0.1:
+core-js@^3.0.1, core-js@^3.6.5:
   version "3.6.5"
   resolved "https://registry.yarnpkg.com/core-js/-/core-js-3.6.5.tgz#7395dc273af37fb2e50e9bd3d9fe841285231d1a"
   integrity sha512-vZVEEwZoIsI+vPEuoF9Iqf5H7/M3eeQqWlQnYa8FSKKePuYTf5MWnxb5SDAzCa60b3JBRS5g9b+Dq7b1y/RCrA==