Fixed: The default duration setting for custom defined transitions was not working in Safari and Firefox

[yhatt]

---
_class: invert
transition: dissolve
style: |
  @keyframes marp-transition-dissolve {
    from {
      --marp-transition-duration: 2s;
  
      opacity: 1;
    }
  
    to {
      opacity: 0;
    }
  }
---

# Slide 1

---

# Slide 2

Marp CLI's bespoke template should detect an initial --marp-transition-duration in the custom defined keyframes for transitions. However, Safari and Firefox Nightly had not detected that.