Reader: Implement new cells

[kean]

This PR adds the initial implementation of the new post cells for the Reader. The new cells use smaller vertical space, have better accessibility support for both dynamic type and voice over, and has new menus.

There is still a little bit more work left, but it's mergable and it's completely under the new feature flag.

Changes

Before and After

The vertical size usage comparison:

Dynamic Type

Context Menus

To test:

• Context menu actions still work

Regression Notes

1. Potential unintended areas of impact

2. What I did to test those areas of impact (or what existing automated tests I relied on)

3. What automated tests I added (or what prevented me from doing so)

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]

	1 Warning
:warning:	This PR is larger than 500 lines of changes. Please consider splitting it into smaller PRs for easier and faster reviews.

Generated by :no_entry_sign: Danger

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

	App Name	WordPress Alpha
	Configuration	Release-Alpha
	Build Number	pr23670-f338663
	Version	25.4
	Bundle ID	org.wordpress.alpha
	Commit	f338663f7b6549d633e539306b38546aae516d85
	App Center Build	WPiOS - One-Offs #10823

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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	pr23670-f338663
	Version	25.4
	Bundle ID	com.jetpack.alpha
	Commit	f338663f7b6549d633e539306b38546aae516d85
	App Center Build	jetpack-installable-builds #9865

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[kean]

I had to rebase but I put the PR comment updates in a separate commit.