[css-2025][css-transitions-2] Add `transition-behavior`, `@starting-style`, and `CSSStartingStyleRule` to Safe to Release exceptions

[SebastianZ]

Splitting the discussion from #10687 for adding the transition-behavior property, @starting-style rule and CSSStartingStyleRule interface of CSS Transitions 2 to the Safe to Release pre-CR Exceptions section of the CSS Snapshot 2025.

Data to consider:

• Spec. status: WD (last published: 5 September 2023)
• Implemented in all major browsers as part of Interop 2024
• WPT: passing tests: Chrome: 60, Edge: 60, Firefox: 59, Safari: 60; interop: 59 / 60 (98.3%)
• Open issues: on GitHub: 6, in spec.: 0

Summary: Very high interoperability, few open issues

Sebastian