feat: measure identified events

[robbie-c]

Changes

Send $is_identified property as a boolean, based on whether identify has been called

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!)

[posthog-bot]

Hey @robbie-c! 👋
This pull request seems to contain no description. Please add useful context, rationale, and/or any other information that will help make sense of this change now and in the distant Mars-based future.

[github-actions]

Size Change: +216 B (0%)

Total Size: 944 kB

Filename	Size	Change
dist/array.full.js	225 kB	+54 B (0%)
dist/array.js	124 kB	+54 B (0%)
dist/es.js	124 kB	+54 B (0%)
dist/module.js	125 kB	+54 B (0%)

ℹ️ View Unchanged

Filename	Size
dist/exception-autocapture.js	12.1 kB
dist/recorder-v2.js	106 kB
dist/recorder.js	106 kB
dist/surveys-module-previews.js	62 kB
dist/surveys.js	58.3 kB

_{compressed-size-action}

[pauldambra]

LGTM