Add hideSelectedOptions prop for multi-select components

Overview

This PR adds a new hideSelectedOptions prop to the Vue Select component that allows controlling whether selected options appear in the dropdown menu when using multi-select mode.

Features

• Added new hideSelectedOptions boolean prop (default: true)
• When true (default), selected options are hidden from the dropdown in multi-select mode
• When false, selected options remain visible in the dropdown
• Enhanced the option slot with additional context properties (index, isFocused, isSelected, isDisabled)
• Improved option selection behavior in multi-select mode to toggle options when clicked

Documentation

• Added documentation for the new prop in docs/props.md
• Updated slot documentation in docs/slots.md to include the new slot props

Tests

Added comprehensive tests to verify the behavior:

• Hiding selected options when hideSelectedOptions is true
• Showing selected options when hideSelectedOptions is false
• Confirming single-select behavior remains unchanged
• Verifying deselected options reappear in the dropdown
• Testing search functionality with hidden options

Related Issues

Closes #233