package.json: pin @babel/core to less than 7.12.10

Workaround for #2620

Author: DeeDeeG

package.json

@@ -41,6 +41,7 @@
   "dependencies": {
     "@atom/babel-plugin-chai-assert-async": "1.0.0",
     "@atom/babel7-transpiler": "1.0.0-1",
+    "@babel/core": "7.x <7.12.10",
     "@babel/generator": "7.8.0",
     "@babel/plugin-proposal-class-properties": "7.8.0",
     "@babel/plugin-proposal-object-rest-spread": "7.8.0",