[Internal] Configure babel-plugin-private-methods loose explicitly

[dac09]

What?

As title

Why?

When running yarn build, we got warnings from babel. To reproduce, in main, run build in the cli package.

cd packages/cli
yarn build

you should see warnings like this:

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-methods", { "loose": true }]
to the "plugins" section of your Babel config.
Though the "loose" option was set to "false" in your @babel/preset-env config, it will not be used for @babel/plugin-proposal-private-methods since the "loose" mode option was set to "true" for @babel/plugin-proposal-class-properties.
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

[peterp]

I always thought stage-4 proposals were added automatically, are they, but because we have loose: true on the others we need to do these for all class related proposals?

[dac09]

The preset-env does add a bunch of plugins automatically, but I think this is mainly that dependency between class and private methods plugins.