preset-env: Offer automated locus of control to consumers for `targets` choices (and whether to include builtin deps.)

## Feature Request

**Is your feature request related to a problem? Please describe.**

While `@babel/preset-env` has `useBuiltIns` (and `exclude: ["@babel/plugin-transform-regenerator"]`) to reduce what is consumed out of core-js (or regenerator-runtime), libraries are still forced to determine targets and consumers are all still forced to use `core-js` or `regenerator-runtime` (and have no choice as to their own wishes for targets). OTOH, when `useBuiltins` is not used, libraries or applications are unaware of what dependencies ought to be added or having an automated way to get at this, at least without going through another Babelification routine.

**Describe the solution you'd like**

I am thinking Babel (as with `@babel/preset-env`) could optionally save a config file indicating the polyfills for builtins in use within a package (though bundling it separately from the distribution file, say as JSON).

A build tool employed optionally by an application could look for this config file in each of its dependencies and add `core-js` or `regenerator-runtime` imports to the application's entry file (eliminating duplicate import statements if different projects required the same).

**Describe alternatives you've considered**

(Explained above.)

**Teachability, Documentation, Adoption, Migration Strategy**

Libraries using `@babel/preset-env` could set a new option `buildBuiltinsJSON` to `true` and probably avoid setting `useBuiltIns`.

Consuming applications would ideally have access to a command line or programmatic tool from Babel to add (or replace existing) imports within a specified entry file for their application, based on the `.babel-builtins.json` files across all of their dependencies.

No repository file or test is named. Start by reviewing @babel/preset-env's useBuiltIns, targets, and exclude behavior, then consider how an optional .babel-builtins.json would describe dependencies for an application's entry file. Done would include the requested configuration output and a CLI or programmatic way to apply the resulting core-js or regenerator-runtime imports.

Written by the indexing model from the issue text.