feat: allow ignoring languages to fix conflicts with image renderers

[bezlant]

I've been monkey-patching this into my config for a while to solve a conflict with snacks.nvim.

When rendering diagrams (like mermaid), render-markdown conceals the code block ticks and applies its own styling, which prevents snacks from correctly detecting and rendering the diagram image.

This PR adds an ignore list to the code config (e.g., ignore = { "mermaid" }). If a language is in this list, the plugin will skip rendering it entirely, allowing other tools to handle it without interference.

Please let me know if you think this is a good addition or if there's a better way to handle these conflicts