Typing mixins with JSDoc

Nobody has claimed this yet.

Consider having JSDoc examples for typing mixins. Something along the lines of the following has been working for me.

Create a dummy class that will be used as the interface:
```js
export class MyMixinInterface {
/** @returns {Promise} */
renderHighlight() {
return Promise.resolve();
}
}
```

And then use it in the mixin:

```js
/**
* @template TBase
* @param {TBase & {new (...args: any[]): LitElement}} baseElement will be the class from which it is extended
* @returns {TBase & {new (...args: any[]): import('./myMixinInterface').MyMixinInterface}} baseElement with mixins applied
*/
export const myMixin = (baseElement) => class extends baseElement {
renderHighlight() {
/* ... */
}
};
```

Hopefully I've not made any typos :-)

VSCode and TypeScript seems to work well. VSCode intellisense autocomplete being displayed and no TS warnings/errors.

Review the JSDoc mixin examples in the issue and inspect the repository's existing documentation entry points for the appropriate location. Done means the documentation includes a clear example of typing mixins and preserves the VSCode and TypeScript autocomplete and error-checking behavior described.

Written by the indexing model from the issue text.