feat(markdown): Expose default components via Default prop

[natemoo-re]

Each MarkdownComponents override now receives a Default prop — a reference to the built-in component for that element. Consumers can conditionally customize rendering and fall through to the default without importing the internal defaultComponents module.

<Markdown
  raw={content}
  components={{
    Heading: ({children, level, Default}) => {
      if (level > 3) return <strong>{children}</strong>;
      return <Default level={level}>{children}</Default>;
    },
  }}
/>

Image is excluded since it has no built-in default. Two styled table cell components (DefaultTableHeaderCell, DefaultTableCell) need a cast due to native HTML align attribute narrowing.

[github-actions]

📊 Type Coverage Diff

✅ No new type safety issues introduced. Coverage: 93.56%

[priscilawebdev]

Apart from the null/undefined types that we still need to fix, this looks good to me. I'm new to this project, so it might be worth getting one more approval.