[field] add focus support for multiple input fields inside of a slot

[Lillious]

Describe the feature: Currently focus support is only available for single input slot elements and does not properly work with components such as date-range-picker. This feature would allow for multiple input fields to have separate focus attributes.

Additional context: We could update the _onToInputFocus method to add an optional target of HTMLInputElement to be able to determine which element inside the slot is currently being focused.