feat(treesitter): vertical conceal support for highlighter

TSHighlighter now places marks for conceal_lines metadata. A new
internal decor provider callback _on_conceal_line was added that
instructs the highlighter to place conceal_lines marks whenever the
editor needs to know whether a line is concealed. The bundled markdown
queries use conceal_lines metadata to conceal code block fence lines.

src/nvim/api/extmark.c

@@ -1049,6 +1049,7 @@ void nvim_set_decoration_provider(Integer ns_id, Dict(set_decoration_provider) *
     { "on_end", &opts->on_end, &p->redraw_end },
     { "_on_hl_def", &opts->_on_hl_def, &p->hl_def },
     { "_on_spell_nav", &opts->_on_spell_nav, &p->spell_nav },
+    { "_on_conceal_line", &opts->_on_conceal_line, &p->conceal_line },
     { NULL, NULL, NULL },
   };