Allow `data` function to return a promise

[srmagura]

Fixes #314.

If the data function returns a Promise or other thenable, the suggestions are updated when the promise resolves.

The type definitions already have Promise as a valid return type for DataFunc, so this PR brings the actual functionality in line with what you would expect from the types.

I added tests for both types of async data functions, callback-based and promise-based.

I have the Prettier extension enabled and it made some formatting changes. If necessary, I can disable the extension and remake my changes so the diff is as small as possible.

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: ea8a670e1e5e6cdfa7b3023fef41e6509550fb3a

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
react-mentions	Minor

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Name	Status	Preview	Updated
react-mentions	✅ Ready (Inspect)	Visit Preview	Jul 22, 2022 at 2:40PM (UTC)

[nikeee]

Any chance to get this merged?