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

[ecbrodie]

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).

I hope this suggestion is helpful.