Hi,

this PR adds an option to allow a single extend (option: allowSingleExtend: true|false) to the rule no-empty-interface. Our use case for this is the following:

Use case

We are building a React application with TypeScript. In React, you typically have components which might look like this:

const ProductTile<ProductTileProps> = () => {
  // Component implementation
}

Problem

Let's say our <ProductTile> component accepts the exact same props as the members defined in an interface Product { ... }. We could change the definition of the <ProductTile> component as follows to support that:

const ProductTile<Product> = () => {
  // Component implementation
}

But once we have to add an additional prop (e.g. onClick?(event: MouseEvent<HTMLButtonElement>): void), we have the issue that we would have to extend Product (which doesn't make sense as onClick doesn't belong to a Product) or replace the interface.

Solution

Instead, we would prefer to define the component as follows initially:

interface ProductTileProps extends Product {}

const ProductTile<ProductTileProps> = () => {
  // Component implementation
}

Now if we add an additional prop like onClick, we could simply add it to the interface ProductTileProps and we're done:

const ProductTileProps extends Product {
  onClick?(event: MouseEvent<HTMLButtonElement>): void
}

// Component stays 100% the same

This PR allows to do that while still warning if empty interfaces like interface Foo {} are used. It's an optional configuration property so that the default behavior is not changed.

P.S.: This is my first PR to change an ESLint-style rule. Let me know if there is anything I should do differently. Thanks a lot!