Implement View Transitions feature plugin

[felixarntz]

This is a follow up issue to #1963, with the purpose to track the actual implementation of the new View Transitions feature plugin up until the initial launch.

The work will be broken down into a few iterative PRs to ease review and discussion. Here's the list:

MVP (launch as experimental)

• Foundation (add_theme_support, opt in to view transitions via CSS) (Scaffold View Transitions feature plugin #1998)
• Add support for specifying global transition names and post transition names (Add support for dynamic view transition names for global elements, and certain post elements to View Transitions plugin #1999)
• Add support for different animations (Add support for different view transition animations for the default transition type #2016)
• Add settings UI to control animations, global transition names, and post transition names (Implement basic settings for View Transitions plugin #2025)
• Add plugin assets for wordpress.org (Add View Transitions plugin assets for wordpress.org #2026)
• Mark plugin as ready to launch and include in Performance Lab (Prepare View Transitions for launch as part of Performance Lab program #2027)
• Publish first version on WordPress.org

Polishing (graduation to non-experimental)

• Add support for customizing transition animation duration
• Add support for chronological and pagination transitions

The implementation of all these PRs will be based on WordPress/wordpress-develop#8370 and heavily reuse the code from that experimental Core PR. The main difference will be the inclusion of settings to control via UI, plus adjustments to satisfy requirements of implementing as a plugin.

[felixarntz]

I just deployed the experimental 1.0.0 release, it's now live in the plugin directory.

Note that due to a bug in our workflow (which I reported in #2032), I did a manual deployment of this version. With WordCamp Europe around the corner, this will be a great opportunity to share the project more broadly and ask for feedback.

[felixarntz]

I just deployed a 1.0.1 (now via workflow, since updates work) due to a rather critical bug/limitation that @Soean discovered and fixed in #2036. 🎉