add support for set_once

[alexleventer]

What does this PR do? Adds support for mixpanel.people.set_once (https://developer.mixpanel.com/docs/javascript-full-api-reference#mixpanelpeopleset_once)

Are there breaking changes in this PR? No

Testing Mocha tests added

Any background context you want to provide? Adds support for set properties once on a Mixpanel person.

Is there parity with the server-side/android/iOS integration components (if applicable)? N/A

Does this require a new integration setting? If so, please explain how the new setting works Yes, setOnceProperties (array)

Links to helpful docs and other external resources https://developer.mixpanel.com/docs/javascript-full-api-reference#mixpanelpeopleset_once

[alexleventer]

Looks like https://segment.atlassian.net/wiki/spaces/GRC/pages/453935287/Reinforcing+Change+Control is a private link, unable to access

[alexleventer]

Hey @dielduarte - do you how the process works to get PRs reviewed by Segment?