feat(dropdown): support TypeScript generics

[metonym]

Supports #2285

Related #2271

DataTable already supports generics for row data, which greatly enhances DX (local IDE typeahead, type safety, etc..).

This PR updates Dropdown with a similar feature. It now supports TypeScript generics on items, allowing type-safe access to custom item properties in callbacks, events, and slots.

This is highly useful when storing/passing additional properties on items.

This change should be backward compatible; omitting the generic type parameter defaults to DropdownItem.

Example Usage

The price and category fields are automatically inferred.

<script lang="ts">
  import { Dropdown } from "carbon-components-svelte";

  const items = [
    { id: "1", text: "Laptop", price: 999, category: "Electronics" },
    { id: "2", text: "Phone", price: 599, category: "Electronics" },
    { id: "3", text: "Desk", price: 299, category: "Furniture" },
  ];
</script>

<Dropdown
  {items}
  selectedId={undefined}
  label="Choose a product"
  titleText="Products"
  on:select={(e) => {
    console.log("selected:", e.detail.selectedItem);
  }}
  let:item
>
  {@const { text, price, category } = item}
  <div>
    <strong>{text}</strong> - ${price}
    <span>({category})</span>
  </div>
</Dropdown>