Add "$is_initial_event" to capture events

[benjackwhite]

We have two deprecated plugins that use expensive caching to try and determine if the event is the first one:
https://github.com/posthog/first-event-today
https://github.com/posthog/first-time-event-tracker

I realised, for the user-side of things we could offload the expensive caching to the clients which now becomes relatively cheap.

Todo this we would need to store something like a map of events and timestamps:

eventTimeMaps: {
  "$pageview": Date.now(),
  "custom-event": Date.now()
}

Then everytime we capture event the logic is pseudo:

if (!eventTimeMaps[event.event]) {
  properties["$is_first_event"] = true
  eventTimeMaps[event.event] = Date.now()
} else if (eventTimeMaps[event.event].date < Date.now().date) {
  properties["$is_first_event_today"] = true
  eventTimeMaps[event.event] = Date.now()
}