In preperation for using the `Dropdown` component with the tag farcaster channel modals, I needed to extend the `Dropdown` component. Here is everything new with the `Dropdown` component
1. Introduced `defaultLabel` prop. This prop is the text/label displayed to the user in the dropdown if no option is currently selected
2. `activeSelection` prop is now optional. There will be some cases where initially a dropdown won't have a selection, so we should make this prop optional
3. Introduced `selectedIndex` state. Rather than iterating through the list of options every time the `activeSelection` changes, we can keep track of the index of the `activeSelection` in the list of `options` and use that to immediately get the active displayed option
4. `dropdownList` is now positioned with `absolute` rather than `relative`. The dropdown list was causing the modal to change size whenever the dropdown was opened or closed. With the `dropdownList` using `absolute` position, the dropdown list won't affect the size of the modal. One thing to point out is now there could be overflow with the dropdown list; however this seems to be an acceptable pattern in other places that use a dropdown component
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