Description

This PR addresses issue #203 by introducing a CustomModifier trait that makes implementing custom modifiers much easier and more intuitive.

Problem

The current typeclass encoding for custom modifiers is verbose and not straightforward to understand:

case class DataAttribute(value: String)
given Modifier[IO, dom.Element, DataAttribute] = (attr, elem) =>
  Resource.eval(IO.delay(elem.setAttribute("data-custom", attr.value)))

Solution

Added a CustomModifier[F, E] trait that provides a simpler API:

case class DataAttribute(value: String) extends CustomModifier[IO, dom.Element]:
  def apply(element: dom.Element): Resource[IO, Unit] =
    Resource.eval(IO.delay(element.setAttribute("data-custom", value)))

Changes

• ✅ Added CustomModifier[F, E] trait with automatic typeclass derivation
• ✅ Added sync helper method for simple synchronous operations
• ✅ Comprehensive test suite covering all use cases
• ✅ Complete documentation with examples and migration guide
• ✅ Example code demonstrating the improvement

Benefits

• Simplified API: Much more intuitive than manual typeclass implementation
• Automatic Derivation: No need to write given instances manually
• Better Developer Experience: Cleaner, more readable code
• Backward Compatible: Existing code continues to work unchanged
• Zero Breaking Changes: Pure additive enhancement

Testing

All tests pass and demonstrate:

• Basic custom modifier usage
• Integration with HTML DSL
• Combining with other modifiers
• Using the sync helper method
• Complex conditional logic
• Comparison with the old approach

Fixes #203