[css-shadow-parts] Expose parts programmatically

It would be helpful for components with parts to expose those parts in some way that's programmatically inspectable.

For components with an open shadow root, someone could query inside the shadow for elements that have a part attribute, but this is not possible for components with closed shadow roots. To avoid leaking shadow elements, the proposed mechanism would return an array of string part names, not elements.

This would be akin to property reflection, and good for things like design/debugging/testing tools. E.g., it'd be nice if a tool could ask a DOM element what parts it has so that the tool can show the user a list of the parts available for styling.

Using the spec's example:

<template id="c-e-template">
  <span part="textspan">This text will be red</span>
</template>

The goal would be to allow something like:

const e = document.createElement('custom-element');
e.parts; // Returns the array ['textspan']

No repository file or test is identified in the issue. Start by reviewing the CSS Shadow Parts specification and the open discussion, then define the programmatic parts API and its behavior for closed shadow roots. Done means the proposal is specified clearly enough for tools to retrieve an element's part names without exposing shadow elements.