Add keyboard navigation to multi-select components

[mikiher]

This (partially) fixes #2555

Adds support for keyboard navigation and selection in the dropdown menu of multi-select components.

Details:

• Support is added for ArrowDown and ArrowUp for navigation
  • ArrowDown/Up highlights the next/previous menu item
• By default, no menu item is highlighted
  • If no menu item is highlighted, Enter simply adds the current text input to the selected list
  • If a menu item is highlighted, Enter adds/removes the highlighted item to/from the selected list
• Highlighted menu element is marked with text-sky-400 (blue text color)
• Menu scrolling is properly supported
  • After ArrowDown/Up, scroll position is modified to keep the highlighed item visible.
• Mouse navigation and selection continues to work normally as expected

Notes:

1. Using blue text color for highlighting seemed like a reasonable choice, but I'm not a designer - I'll change it to whatever you think is best.
2. There's some refacroring work required to make this code more reusable. I'll do it in a future PR.

[advplyr]

I updated the highlight to yellow-300 in keeping with the theme color. It is working great for me, thanks!