Move package.json exports support to a config option

[devongovett]

Unfortunately enabling package.json "exports" field support by default would be a breaking change. This is due to many reasons, two of which are:

1. Un-exported paths would suddenly fail to resolve where they did not before.
2. require and import could now resolve to different modules due to export conditions, resulting in broken singletons (e.g. React context).

Therefore, until the next major version, package exports support will be opt-in by adding the following to your project root package.json:

{
  "@parcel/resolver-default": {
    "packageExports": true
  }
}