Add Node#fence for code block metadata

[kristoph]

Adds Markly::Node#fence to expose the parsed marker metadata for fenced code blocks.

The method returns a Markly::Node::Fence structure containing:

• character — the backtick or tilde marker.
• length — the number of marker characters.
• indent — the indentation before the opening fence.

It returns nil for indented code blocks and non-code nodes. The code info string remains available separately through Node#code_info.

block = Markly.parse("  ~~~~ ruby\n  Object.new\n  ~~~~").first_child

block.fence
# => #<struct Markly::Node::Fence character="~", length=4, indent=2>

The structure is defined and instantiated directly by the native extension. This change also adds coverage for backtick and tilde fences, non-default fence lengths and indentation, indented code blocks, and non-code nodes.

[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.