feat: Add person processing mode preview

[robbie-c]

Enable by setting __preview_process_person in config

Changes

See PostHog/posthog.com#8098

• Add init config option
• Pass flag as event property
• Ensure that $set etc are not sent with personless events (no-op, they were only sent with identify afaict)
• Ensure that identify cannot be called if person mode is never
• Ensure that we send referrer and campaign props with all events with person processing when in identified_only mode

Manual tests

• identified_only sets up initial_utm properties etc correctly

Checklist

• Tests for new code (see advice on the tests we use)
• Accounted for the impact of any changes across different browsers
• Accounted for backwards compatibility of any changes (no breaking changes in posthog-js!)

On this last one, the default person processing mode is always which is identical to the current behaviour

[github-actions]

Size Change: +2.25 kB (0%)

Total Size: 947 kB

Filename	Size	Change
dist/array.full.js	225 kB	+107 B (0%)
dist/array.js	126 kB	+1.95 kB (+2%)
dist/es.js	127 kB	+1.95 kB (+2%)
dist/module.js	127 kB	+1.95 kB (+2%)
dist/recorder-v2.js	105 kB	-1.84 kB (-2%)
dist/recorder.js	105 kB	-1.84 kB (-2%)

ℹ️ View Unchanged

Filename	Size
dist/exception-autocapture.js	12.1 kB
dist/surveys-module-previews.js	62 kB
dist/surveys.js	58.3 kB

_{compressed-size-action}

[pauldambra]

it's a three-glasses-of-prosecco, saturday evening review, treat it accordingly 🤣

I've not really been following this project so I'm not 100% but I think this does the right thing

[pauldambra]

sometimes I wonder if the bundler is random 🤣

weird that we've apparently shifted code from recorder to core 🤯

maybe we should handle separately since this needs to ship I guess.

[robbie-c]

sometimes I wonder if the bundler is random @pauldambra

Yeah - working with optimizers is a pain for exactly this reason.