MDCList notifyAction() emits primitive event detail

[acdvorak]

Problem

We're currently emitting a number value for the CustomEvent.detail property in
MDCListAdapter.notifyAction():

material-components-web/packages/mdc-list/index.js

Lines 279 to 281 in af950fc

	notifyAction: (index) => {
	this.emit(strings.ACTION_EVENT, index, /** shouldBubble */ true);
	},

This is technically allowed by TypeScript's CustomEvent<T> interface:

https://github.com/Microsoft/TypeScript/blob/2f9218f346bbc606cccc4aebfca1c2c4f3167fee/lib/lib.dom.d.ts#L3415-L3422

interface CustomEvent<T = any> extends Event {
    /**
     * Returns any custom data event was created with.
     * Typically used for synthetic events.
     */
    readonly detail: T;
    initCustomEvent(typeArg: string, canBubbleArg: boolean, cancelableArg: boolean, detailArg: T): void;
}

However, emitting a primitive value for an event detail is not future-resistant. If we ever need to pass more data, we'll be forced to break backward compatibility for a large number of customers.

Solution

Rather than break backward compatibility later—after lots of downstream consumers have already wrapped the new v0.44.0 MDCList API from PR #4144—we should break it now.

This will also simplify PR #4334 and PR #4342.