Allowed children for slots

Both standard components (e.g. `table`, `tr`, `td`, `dl`, `dd`) and web components (e.g. `tab-panels`, `tab-panel`) have implicit constraints on what elements are allowed as children, and also what are allowed as parents.

I would propose that a slot description should also include a way to codify this.

For example:

```ts
export interface Slot {
/**
* The slot name, or the empty string for an unnamed slot.
*/
name: string;

/**
* A markdown summary suitable for display in a listing.
*/
summary?: string;

/**
* A markdown description.
*/
description?: string;

/**
* A set of CSS selectors for allowable children
*/
allowedChildren?: string[];
}
```