Simplify reaction_added/removed event's item type

[seratch]

Summary

This pull request improves TypeScript developer experience by updating "item" property type in reaction_added/removed event data types. WIth this change, developers can safely access event.item.channel in TS code. More importantly, they can use say() utlitity in listeners. Currently, say() can be resolved as never in TS as event.item.channel can be absent for file/file comment items while it never happens. Slack apps no longer receive reaction events for files and file comments. So, this change is safe enough to apply in a patch release.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

Merging #1845 (817dc85) into main (6eaf5f8) will not change coverage.
The diff coverage is n/a.

❗ Current head 817dc85 differs from pull request most recent head 13ce4b2. Consider uploading reports for the commit 13ce4b2 to get more accurate results

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  Hits         1248     1248           
  Misses        175      175           
  Partials       96       96           

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[filmaj]

LGTM! Left one minor comment