feat: Improve editor session events

[dcalhoun]

• Introduce gutenberg_kit editor session event property value.
• Track editor session start event for GutenbergKit sessions.

Related:

• https://github.com/Automattic/dotcom-forge/issues/8268.
• feat: Introduce gutenberg_kit editor session property WordPress-Android#21564
• feat: Communicate editor load event to host app GutenbergKit#54

To test:

1. Open the experimental editor.
2. Close the editor.
3. Verify editor_session_start|end events trigger with a
   editor: gutenberg_kit property.

Regression Notes

1. Potential unintended areas of impact
   Disrupt events for the other editors.
2. What I did to test those areas of impact (or what existing automated tests I relied on)
   Verify the editor session events work as expected.
3. What automated tests I added (or what prevented me from doing so)
   Deemed unnecessary for this experimental editor analytic event.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding unit tests for my changes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Testing checklist:

• WordPress.com sites and self-hosted Jetpack sites.
• Portrait and landscape orientations.
• Light and dark modes.
• Fonts: Larger, smaller and bold text.
• High contrast.
• VoiceOver.
• Languages with large words or with letters/accents not frequently used in English.
• Right-to-left languages. (Even if translation isn’t complete, formatting should still respect the right-to-left layout)
• iPhone and iPad.
• Multi-tasking: Split view and Slide over. (iPad)

[dangermattic]

	2 Warnings
⚠️	Modules/Package.swift was changed without updating its corresponding Package.resolved. Please resolve the Swift packages as appropriate to your project setup (e.g. in Xcode or by running swift package resolve).
⚠️	PR is not assigned to a milestone.

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	Build Number	pr23955-283ee9d
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	Bundle ID	org.wordpress.alpha
	Commit	283ee9d
	App Center Build	WPiOS - One-Offs #11305

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[wpmobilebot]

📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr23955-283ee9d
	Version	25.6
	Bundle ID	com.jetpack.alpha
	Commit	283ee9d
	App Center Build	jetpack-installable-builds #10338

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