Scaffold View Transitions feature plugin

[felixarntz]

Summary

See #1997.

Relevant technical choices

• This is the first of several PRs to implement the first version of the plugin.
• This PR only scaffolds the basics and opts the site in to using view transitions, controlled via theme support.
• Subsequent PRs will add more advanced functionality, like view transition names, different transition types, and different animations.
• This PR does not add the plugin to plugins.json, since it should not be released yet until all work for Implement View Transitions feature plugin #1997 has been completed.

[github-actions]

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Co-authored-by: felixarntz <[email protected]>
Co-authored-by: swissspidy <[email protected]>

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[codecov]

Codecov Report

Attention: Patch coverage is 76.92308% with 3 lines in your changes missing coverage. Please review.

Project coverage is 72.52%. Comparing base (ff8b62b) to head (f75b6b1).
Report is 3 commits behind head on trunk.

Files with missing lines	Patch %	Lines
plugins/view-transitions/hooks.php	25.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##            trunk    #1998   +/-   ##
=======================================
  Coverage   72.51%   72.52%           
=======================================
  Files          85       87    +2     
  Lines        7023     7036   +13     
=======================================
+ Hits         5093     5103   +10     
- Misses       1930     1933    +3     

Flag	Coverage Δ
multisite	72.52% <76.92%> (+<0.01%)	⬆️
single	40.40% <76.92%> (+0.06%)	⬆️

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[mukeshpanchal27]

Should be 1.0.0

[felixarntz]

Thanks @mukeshpanchal27, I'll fix this as part of #1999.