Normalize native extension naming

[samuel-williams-shopify]

Summary

• Rename Markly's native wrapper identifiers to mirror their Ruby constant paths, following the conventions used by io-event.
• Name typed-data descriptors and callbacks consistently as Markly_Node_Type, Markly_Node_Type_mark, and related forms.
• Use conventional suffixes such as _allocate and _p where appropriate.
• Group class registration into Init_Markly_Node and Init_Markly_Parser helpers.

This is an internal naming and organization refactor; it does not change the Ruby API.

Testing

• Built the native extension with ./extconf.rb && make -j4.
• Ran a native smoke test covering extension discovery, parser construction, table parsing and rendering, Node#fence, and Node.new.