feat(flags): Add local props and flags to all calls

[neilkakkar]

Problem

Copy of PostHog/posthog-python#106

This turned out to be a lot trickier than expected, because I noticed a few issues. With the sendFeatureFlags capture option, it seems like we were dropping events, since on shutdown the feature flag promises aren't tracked for populating flag information, which means the capture is not enqueued.

I can't yet think of a better way to solve this right now, so we just start tracking flag promises from within capture as well, annnd flush again on shutdown after pending promises have resolved, to make sure all new capture events queued are sent as well.

I'm also moving a few more things that should've been based on the onError emitter, and fixed tests that incorrectly assumed this was working fine.

(cc: @marandaneto @benjackwhite have a look here too please?)

Changes

Release info Sub-libraries affected

Bump level

• Major
• Minor
• Patch

Libraries affected

• All of them
• posthog-web
• posthog-node
• posthog-react-native

Changelog notes

• Added support for X

[neilkakkar]

Missing a test for adding default properties to all flag functions

[neilkakkar]

oh much nicer, thanks!! 😓