Add Quirion Dryad

[radar]

Summary

• Implements Quirion Dryad (Core Set 2021 reprint): 1/1 Elf Druid for {1}{G}.
• Whenever you cast a white, blue, black, or red spell, put a +1/+1 counter on Quirion Dryad.
• Uses TriggeredAbility::SpellCast and gates on you? plus (spell.colors & [:white, :blue, :black, :red]).any?.

Closes #208

Test plan

• Triggers on white/blue/black/red spell cast by controller
• Does not trigger on green or colorless spells
• Does not trigger on opponent's spells
• Full RSpec suite remains green

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