SUGGESTION: Modify useBuiltIns for babel-preset-env in babelrc

The supported values for the `useBuiltIns` option of `babel-preset-env` config changed from Babel v6 to Babel v7. It used to be just a boolean in v6. However, in v7, the supported values are `false`, `"usage"` or `"entry"`. (https://babeljs.io/docs/en/babel-preset-env#usebuiltins)

I suggest that babel-upgrade supports automatically converting an option of `useBuildIns = true` to one of `"usage"` or `"entry"`. This helps remove a manual operation that the developer will have to do, possibly forget about (since this option change is not mentioned in the [Babel v7 Upgrade Guide](https://babeljs.io/docs/en/v7-migration)).

I hope this suggestion is helpful.

Start by locating the babel-upgrade handling for `.babelrc` and the `useBuiltIns` option, then read the linked Babel preset-env documentation and Babel v7 migration guide. Determine whether `true` should map to `"usage"` or `"entry"`; done means the upgrade handles the selected conversion consistently without requiring the manual operation described.

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