Add index for multi select values

[tonytangau]

This change is for users with drag and drop requirements.

[tonytangau]

Created a drag and drop multi select example based on this PR: sortable-select

[ttttsai]

This looks great, could we merge this? @JedWatson @gwyneplaine

[kroeberk]

Please merge this. This is a great and simple implementation.

[JedWatson]

This looks good, thanks @tonytangau (and sorry for the delayed review!)

If you're up for it, adding your example from your sortable-select repo to the docs would be a great addition to help people understand how to use this feature.

Please open a PR for the "advanced" page if you're up for it!

[changeset-bot]

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[gunn]

Glad we have this in now.
@JedWatson my PR, which this one comes from, has an example added to the docs too - #3645

[JedWatson]

Hey @gunn yeah, I found that after merging this :) working to pull your PR into master now, thanks for that!

[gunn]

Awesome. Thank you @JedWatson.

[loopmode]

Hey guys. Thanks for adding the index, it really helps! (And not only in drag+drop scenarios :))

Not sure whether this is the right place This is probably the wrong place... but it would be great to add this to @types/react-select as well! (in MultiValue.d.ts)
Is somebody here affiliated with the types package?