Add Node#fenced method for code block fence metadata

[kristoph]

Adds Markly::Node#fenced method to expose fence metadata for code block nodes.

For fenced code blocks, returns a hash with:

• :info — the info string (e.g., "ruby" in ```ruby)
• :length — fence length (number of backticks/tildes)
• :character — the fence character ("`" or "~")
• :offset — the indentation offset

Returns nil for indented code blocks or non-code-block nodes.

Example

doc = Markly.parse("```ruby\nputs 'hi'\n```")
doc.walk do |node|
  if node.type == :code_block && (fence = node.fenced)
    puts fence[:info]      # => "ruby"
    puts fence[:character] # => "`"
  end
end

[ioquatix]

I'm not sure about the name fenced. I also wonder if we should use a Hash for this. Is there any precedent in other parts of the code? Maybe it's better to use a Data or Struct? 🤷

However, in principle, I am okay with this change.

[ioquatix]

After thinking about this, what about adding fence_character or fence_marker etc.