feat: move heatmaps to their own event

[pauldambra]

see PostHog/posthog#23395

this moves heatmap data from being a passenger on other events to being its own event

PostHog/posthog#23505 is merged so ingestion can now handle existing passenger events or these new $$heatmap events

just a final round of testing for this SDK side now remains

[vercel]

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Name	Status	Preview	Updated (UTC)
posthog-js	✅ Ready (Inspect)	Visit Preview	Jul 10, 2024 0:42am

[github-actions]

Size Change: +2.39 kB (+0.21%)

Total Size: 1.15 MB

Filename	Size	Change
dist/array.full.js	317 kB	+602 B (+0.19%)
dist/array.js	155 kB	+597 B (+0.39%)
dist/main.js	156 kB	+597 B (+0.39%)
dist/module.js	155 kB	+597 B (+0.39%)

ℹ️ View Unchanged

Filename	Size
dist/exception-autocapture.js	10.4 kB
dist/recorder-v2.js	110 kB
dist/recorder.js	110 kB
dist/surveys-preview.js	59.6 kB
dist/surveys.js	64.7 kB
dist/tracing-headers.js	8.26 kB
dist/web-vitals.js	5.79 kB

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[daibhin]

Couple of clarifying questions but all good otherwise