Adds the options `extends`, which can be defined via configuration file or CLI arguments to a single path or an array of paths of shareable configuration.
A shareable configuration is a file or a module that can be loaded with `require`.
Options is defined by merging in the following order of priority:
- CLI/API
- Configuration file
- Shareable configuration (from right to left)
Options set in a shareable configuration can be unset by setting it to `null` or `undefined` in the main configuration file. If a default value applies to this property it will be used.
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