Add Named Slots support for multi-slot component content projection

[Serbroda]

• Named Slots: Components can now define multiple content areas using <tc:content name="..."> and fill them with <tc:slot name="...">
• Default Slot: Content not wrapped in a tc:slot goes into the unnamed tc:content
• Fallback Content: <tc:content name="x">Default</tc:content> renders fallback when no matching slot is provided
• Backward compatible: Existing components with a single tc:content work unchanged
• 4 new tests: Slot distribution, correct section placement, default override, fallback behavior
• 3 new test templates: named_slots.html, named_slots_default_content.html, named_slots_fallback.html, layout_component.html

Example

  <!-- Component definition -->
  <div th:fragment="card" class="card">
      <div class="card-header"><tc:content name="header"></tc:content></div>
      <div class="card-body"><tc:content></tc:content></div>
      <div class="card-footer"><tc:content name="footer"></tc:content></div>
  </div>

  <!-- Usage -->
  <tc:card>
      <tc:slot name="header"><h1>Title</h1></tc:slot>
      <tc:slot name="footer"><span>Footer</span></tc:slot>
      <p>Body content (default slot)</p>
  </tc:card>