feat: Align GetReactions with Slack API

[olegbespalov]

!BREAKING CHANGE

Previously this method returned only reaction details, but with this change it started to also return the item itself (e.g. message, file, file_comment). This makes this closer to the actual response of Slack API.

To migrate an existing code, you can just use resp.Reactions to access the slice of []ItemReaction.

Pull Request Guidelines

These are recommendations for pull requests.
They are strictly guidelines to help manage expectations.

PR preparation

Run make pr-prep from the root of the repository to run formatting, linting and tests.

Should this be an issue instead

• is it a convenience method? (no new functionality, streamlines some use case)
• exposes a previously private type, const, method, etc.
• is it application specific (caching, retry logic, rate limiting, etc)
• is it performance related.

API changes

Since API changes have to be maintained they undergo a more detailed review and are more likely to require changes.

• no tests, if you're adding to the API include at least a single test of the happy case.
• If you can accomplish your goal without changing the API, then do so.
• dependency changes. updates are okay. adding/removing need justification.

Examples of API changes that do not meet guidelines:

• in library cache for users. caches are use case specific.
• Convenience methods for Sending Messages, update, post, ephemeral, etc. consider opening an issue instead.

[olegbespalov]

Closed in favor of slack-go#1480