What problem does this feature solve?
It's 21st century, to select continuous options we don't want to click click... and click. just bring the browser-native keyboard support for <select> feature here.
What does the proposed API look like?
The Shift key/Ctrl key support is designed to be a default bahavior of Vue <Select multiple>, just like web browsers's default behaviors on HTML <select multiple>. Note that disabled options should not be affected.