feat: add PlaceholderTransitions presets for fade animations

[rainxchzed]

🎯 Goal

Introduce PlaceholderTransitions to simplify placeholder fade-in/fade-out animations with convenient presets.

Currently, developers must manually create animation specs for transitions:

placeholderFadeTransitionSpec = { spring(stiffness = Spring.StiffnessLow) }

This PR adds discoverable presets:

placeholderFadeTransitionSpec = PlaceholderTransitions.smooth

Why this matters: Makes the API more accessible for common use cases while maintaining full customization for advanced needs.

🛠 Implementation Details

Changes:

• Added PlaceholderTransitions object with 6 presets: fast, normal, smooth, slow, snappy, bouncy
• Added custom builders: custom() and tween() for fine-grained control
• Comprehensive KDoc documentation explaining when to use each preset

Design decisions:

1. Named PlaceholderTransitions (not AnimationDefaults) to clearly distinguish from PlaceholderDefaults (which provides highlight effects like shimmer/fade/pulse)
2. Returns () -> FiniteAnimationSpec<Float> to match exact parameter signature
3. Preset values based on Material Design motion guidelines
4. 100% backward compatible - purely additive, no breaking changes

Key distinction:

• PlaceholderDefaults = animated effects on the placeholder (shimmer, pulse, etc.)
• PlaceholderTransitions = how placeholder appears/disappears

✍️ Examples

Basic usage:

Box(
  modifier = Modifier
    .placeholder(
      visible = isLoading,
      highlight = PlaceholderDefaults.shimmer,
      placeholderFadeTransitionSpec = PlaceholderTransitions.fast
    )
)

Different in/out transitions:

Text(
  text = "Loading...",
  modifier = Modifier.placeholder(
    visible = isLoading,
    placeholderFadeTransitionSpec = PlaceholderTransitions.slow,   // Slow fade in
    contentFadeTransitionSpec = PlaceholderTransitions.snappy      // Quick reveal
  )
)

Custom timing:

Image(
  painter = painter,
  contentDescription = null,
  modifier = Modifier.placeholder(
    visible = isLoading,
    placeholderFadeTransitionSpec = PlaceholderTransitions.tween(300)
  )
)

Available presets:

• fast - 200ms tween for quick loading
• normal - Default spring for balanced motion
• smooth - Low stiffness spring for elegant transitions
• slow - 800ms tween for deliberate loading
• snappy - High stiffness spring for instant feel
• bouncy - Low damping spring for playful effect

Demo video:
Screen_recording_20251015_064300.webm

✅ Prepare for Review

./gradlew spotlessApply  # ✅ Completed
./gradlew apiDump        # ✅ Completed

API changes:

• Added new public object: PlaceholderTransitions
• No modifications to existing API
• Binary compatibility maintained