Fix babel 'loose' warning

[jorg-vr]

This pull request explicitly adds @babel/plugin-proposal-private-property-in-object to babel.config.js
We already used it implicitly.

Adding it explicitly allows us to specify the loose: true setting. Again this setting was already used because other babel plugins had this setting.
It solves the warning following warning:

Though the "loose" option was set to "false" in your @babel/preset-env config, it will not be used for @babel/plugin-proposal-private-property-in-object since the "loose" mode option was set to "true" for @babel/plugin-proposal-private-methods.
The "loose" option must be the same for @babel/plugin-proposal-class-properties, @babel/plugin-proposal-private-methods and @babel/plugin-proposal-private-property-in-object (when they are enabled): you can silence this warning by explicitly adding
        ["@babel/plugin-proposal-private-property-in-object", { "loose": true }]
to the "plugins" section of your Babel config.

Closes #3142 .