Implement a `MultiComboBox` and `MultiTextInput`

[seanrcollings]

Here's what I'm imagining for an API for these

<MultiTextInput value="current-input-value">
    <Chip key="key1">Chip 1</Chip>
<MultiTextInput value="current-inpu-value">

I would like to use a more compasable API here as opposed to the ComboBox. Potentially consider re-working the ComboBox as well? If I do that, I'd want to make sure that the old behavior was still an option as the combobox is being used in places

<MultiComboBox>
  <MultiTextInput />
  <Popover>
      <ListBox>
        <Item key="value" />
     </ListBox>
  </Popover>
</MultiComboBox>

[seanrcollings]

adobe/react-spectrum#2140