add support for user_profile_changed callback event

[verygoodsoftwarenotvirus]

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

n/a

This change allows for the Slack library to parse incoming callback events of the user_profile_changed type. Previously, parseInnerEvent threw an error because it didn't have record of this event type.

[kanata2]

👍