[Feat]: Support for multi-select in Combobox and Picker

### Code of conduct

- [x] I agree to follow this project's code of conduct.

### Impacted component(s)

Combobox, Popover

### Description of the requested feature

In addition to the existing functionality available for Picker and Combobox, a `selects="multiple"` option to allow for multi-select.

### Mockups or screenshots

_No response_

### Implementation notes or ideas

_No response_

### Would you like to track this issue in Jira?

- [x] Yes, please tell me the ticket number!

Start by reading the existing Combobox, Picker, and Popover implementations to understand their current single-selection behavior. Define the expected behavior for a selects="multiple" option across Combobox and Picker, including how selected values are represented and displayed. Done means both components support multi-select consistently with appropriate coverage for the new behavior.

Written by the indexing model from the issue text.