Challenges using babel.config.js directly

[chriskrycho]

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. 😅)

[NullVoxPopuli]

Related: #446