fix: react to ConfettiWidget colors updates

[erperejildo]

Why

Fixes flutter_confetti #102 — updating the colors parameter of a ConfettiWidget has no effect.

When the widget is rebuilt with a different colors list (e.g. via setState in the parent), the emitted confetti keeps using the original colors. Two root causes:

1. _ConfettiWidgetState never implemented didUpdateWidget, so the new color list was simply ignored.
2. ParticleSystem stored the colors in a final field read once at construction, so there was no way to update it even if the state noticed the change.

The only workaround available to users was forcing a full widget recreation via a Key derived from the color value (as noted in the issue).

What changed

• lib/src/confetti.dart — _ConfettiWidgetState now implements didUpdateWidget: when widget.colors differs from oldWidget.colors, the new list is forwarded to the ParticleSystem.
• lib/src/particle.dart — _colors is no longer final; added a colors setter so the emission colors can be updated at runtime.
• test/confetti_test.dart — added 3 tests:
  • ParticleSystem emits particles with the color set through the setter.
  • Newly generated particles pick up updated colors after emission already started.
  • ConfettiWidget rebuilds with a different colors list without errors.

Behavior notes

• Newly emitted particles use the updated colors; particles already in flight keep the color they were created with. This is consistent with how the widget already behaves for other per-particle parameters (size, shape, etc.).
• Only the colors parameter is handled for now — it's the only one whose change the issue reports, and the only one that can be updated safely without affecting in-flight particle physics.

How I tested

• flutter test — all tests pass (existing + 3 new).

• Built a local test app with the exact reproduction sample from the issue (blue square, "Toggle Color" / "Play" / "Pause" buttons, colors: [_particleColor]) wired to this branch via a path dependency, and ran it on Chrome:

  1. Initial state: blue square, no confetti.
  2. Click Toggle Color → square turns red (triggers the ConfettiWidget rebuild).
  3. Click Play → confetti is now emitted in red (verified by pixel analysis of screenshots: ~2000 red confetti pixels, zero blue).

  On the released 0.8.0 code the same steps emit blue confetti, matching the issue's video.

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