Challenges using babel.config.js directly

In `ember-modifier`, I attempted to use `babel.config.js` directly, in concert with the new `buildEmberPlugins` API. However, this has a problem in that `@babel/preset-env` requires `targets` to be set, and while it makes sense for addons to supply their own *config*, they should not be supplying their own *targets*: the host should always be in control of the targets it uses to build. Additionally, this raises the question of whether all addons should be installing their own copies of `@babel/preset-env` to make this work (presumably yes? But there's then a further degree of explicit ecosystem coordination required for upgrades). Net, we need further design work here.

(Prompted by a conversation with @rwjblue where he explained all of this to me. 😅)