feat: Add inheritance strategies to style variants

[omarluq]

What is the purpose of this pull request?

resolves #48 & #49

Adds 3 inheritance strategies to style variants:

1. override (current behavior): Completely replaces parent variants.

class Parent::Component < Component
  style do
    variants do
      size {
        md { 'text-md' }
        lg { 'text-lg' }
      }
      disabled {
        yes { "opacity-50" }
      }
    end
  end
end

class Child::Component < Parent::Component
  style do
    variants do
      size {
        lg { 'text-larger' }
      }
    end
  end
end

Component only retains the size variant with lg option

2. merge (deep merge): Preserves all variant keys unless explicitly overwritten.

class Parent::Component < Component
  style do
    variants do
      size {
        md { 'text-md' }
        lg { 'text-lg' }
      }
      disabled {
        yes { "opacity-50" }
      }
    end
  end
end

class Child::Component < Parent::Component
  style do
    variants(strategy: :merge) do
      size {
        lg { 'text-larger' }
      }
    end
  end
end

Maintains access to all variants and keys, with lg key overwritten.

3. extend (shallow merge): Preserves variants unless explicitly overwritten.

class Parent::Component < Component
  style do
    variants do
      size {
        md { 'text-md' }
        lg { 'text-lg' }
      }
      disabled {
        yes { "opacity-50" }
      }
    end
  end
end

class Child::Component < Parent::Component
  style do
    variants(strategy: :extend) do
      size {
        lg { 'text-larger' }
      }
    end
  end
end

Maintains all variants but size variant only retains lg key.

Checklist

• I've added tests for this change
• I've added a Changelog entry
• I've updated a documentation