feat: add global user properties plugin

[kelvin-lu]

Summary

This PR adds a simple enrichment plugin that transforms the user properties field into global_user_properties - a beta feature used internally first

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[kelsonpw]

It seems like there is a new CHANGELOG.md file, but the diff indicates it's empty.

Just a few comments and questions to address, otherwise LGTM.

[kevinpagtakhan]

@justin-fiedler should we point this pr to v1.x?

[kelvin-lu]

for other events which are not belong to amplitude special events, if there is no user_properties, do we want to set it to null?

I think returning the event unmodified would work in these cases. I realize i meant to type as TrackEvent and not Base, I can fix that

isNotSpecialAmplitudeEvent means no event_type is '$identify', isAmplitudeIdentifyEvent means the event_type = '$identify'.

The type narrowing is not what i'm expecting it to do, but ideally I'm only modifying events of type IdentifyEvent and BaseEvent - really, I'm trying to avoid mutating group identify and revenue events, and other special event types that may come along the way

[kelvin-lu]

Thanks everyone for the comments (and sorry for the amount of copy-pasta I left on the ground). I've updated the readme and the tests to include a modules.d.ts that fixes the types.

Unfortunately, ts-jest is having some issues picking up this file, so the tests use a couple of any's. I'll make this a follow up ticket if this isn't a blocker to this pr.

[kelvin-lu]

@justin-fiedler @yuhao900914 I've limited my changes to the plugin package - I'll be merging this if there are no concerns