[mixpanel] Add time_event, people.union, and typedef for init config param

[pnw]

Please fill in this template.

• Use a meaningful title for the pull request. Include the name of the package modified.
• Test the change in your own code. (Compile and run.)
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If changing an existing definition:

• Provide a URL to documentation or source code which provides context for the suggested changes:
• mixpanel.time_event
• mixpanel.people.union
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[dt-bot]

types/mixpanel/index.d.ts

to author (@hjellek). Could you review this PR?
👍 or 👎?

Checklist

• pass the Travis CI test?

[pnw]

This resolves: #19642

[typescript-bot]

Approved by a listed owner. PR ready to merge pending express review by a maintainer.

[hjellek]

Sorry for late answer, the change seems okay.

👍