Add typing to AppMentionEvent files array

[ian]

Summary

This fixes the lack of typing for AppMentionEvent's files array. There's already a note to add it.

See https://github.com/slackapi/node-slack-sdk/blob/039bd050cd736d666d063397067cb014adf23711/packages/types/src/events/app.ts#L136

Requirements (place an x in each [ ])

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

[salesforce-cla[bot]]

Thanks for the contribution! Before we can merge this, we need @ian to sign the Salesforce Inc. Contributor License Agreement.

[misscoded]

Hi @ian! Thanks for the contribution. Could you take a moment to sign the required CLA?