Add Custom Transition Settings to theme.json

[geekofshire]

Summary

This PR adds custom settings for CSS transitions in the theme.json file. The new settings include transition durations, timing functions, and delays, each with detailed descriptions, types, and default values.

Changes

• Added transition object under settings.custom in theme.json.
• Defined properties for duration, timing, and delay with metadata.
• Enhanced documentation within theme.json for better understanding and maintenance.

Impact

These changes allow themes to define default CSS transitions, providing a consistent and customizable user experience across blocks and patterns.

Related Issue - #62885

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[t-hamano]

Thanks for the PR!

You probably submitted this PR after seeing my comment.

However, the code I proposed is not intended to be incorporated as the default into WordPress core/Gutenberg, but merely defined an example of adding custom properties as needed to each theme.

Since the CSS properties we want to add vary depending on the theme, it would be difficult to incorporate hard-coded values ​​as defaults.

Therefore, I will close this PR, but please feel free to comment if there is anything I have missed.