feat: add dropdown component

[FrederikBolding]

Adds support for a Dropdown component, that can be used on its own or as part of a Field.

<Dropdown name="dropdown" value="option3">
  <Option value="option1">Option 1</Option>
  <Option value="option2">Option 2</Option>
  <Option value="option3">Option 3</Option>
</Dropdown>

Similarly to Input, it optionally uses value to determine the initial value/preselected dropdown item. This can also be used to control the input when re-rendering with snap_updateInterface. The dropdown will also trigger InputChangeEvent similarly to an Input.

This PR mostly adds validation and types for this new component as well as some handling for the form state. This will need a follow-up PR in the extension to complete integration. This PR also adds support for selectInDropdown to snaps-jest and uses this new component in the Interactive UI example Snap.

Progresses https://github.com/MetaMask/MetaMask-planning/issues/1575

[FrederikBolding]

@metamaskbot update-pr

[Mrtenz]

Maybe we should support a placeholder and default option here?

[FrederikBolding]

Default option is the same as using value. The underlying DS component (which uses plain HTML) doesn't support a placeholder so I would vote we put that off