Add "disabled" attribute to list items that are disabled

[acc-sandeep]

Please respect maintainers time by filling in these sections. Your issue will likely be closed without this information.

• Vue Version: 2.6.11
• Vue Select Version: 3.16.0

Describe the bug
A clear and concise description of what the bug is.
-> You have already added vs__dropdown-option--disabled class for the list items that are disabled.
Same we need to add disabled attribute for the disabled list items as per the accessibility requirement.
This can be easily understood.
https://github.com/sagalbot/vue-select/blob/master/src/components/Select.vue#L99

Reproduction Link
A link to a reproduction of the bug. This is a huge help.

Steps To Reproduce
Outline the steps to reproduce the bug.

Expected behavior
A clear and concise description of what you expected to happen.
-> disabled attribute should be present on the list items that are disabled.
For eg :
<li :disabled></li>

Screenshots

If applicable, add screenshots to help explain your problem.