feat(neovim): Highlight markdown code blocks

This adds syntax highlighting to code blocks with vim-markdown while
keeping pretty much everything else the same.

home/neovim.nix

@@ -386,6 +386,16 @@ in
           nnoremap <localleader>c <plug>(vimtex-env-change)
         '';
       }
+      {
+        plugin = vim-markdown;
+        config = /* vim */ ''
+          let g:vim_markdown_folding_disabled = 1
+          let g:vim_markdown_conceal = 0
+          let g:vim_markdown_frontmatter = 1
+          let g:vim_markdown_toml_frontmatter = 1
+          let g:vim_markdown_json_frontmatter = 1
+        '';
+      }
       {
         plugin = alpha-nvim;
         type = "lua";