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/buffer_defs.h

@@ -1313,23 +1313,18 @@ struct window_S {
   linenr_T w_statuscol_line_count;      // line count when 'statuscolumn' width was computed.
   int w_nrwidth_width;                  // nr of chars to print line count.
 
-  qf_info_T *w_llist;                 // Location list for this window
+  qf_info_T *w_llist;                   // Location list for this window
   // Location list reference used in the location list window.
   // In a non-location list window, w_llist_ref is NULL.
   qf_info_T *w_llist_ref;
 
-  // Status line click definitions
-  StlClickDefinition *w_status_click_defs;
-  // Size of the w_status_click_defs array
-  size_t w_status_click_defs_size;
+  StlClickDefinition *w_status_click_defs;      // Status line click definitions
+  size_t w_status_click_defs_size;              // Size of the w_status_click_defs array
+  StlClickDefinition *w_winbar_click_defs;      // Window bar click definitions
+  size_t w_winbar_click_defs_size;              // Size of the w_winbar_click_defs array
+  StlClickDefinition *w_statuscol_click_defs;   // Status column click definitions
+  size_t w_statuscol_click_defs_size;           // Size of the w_statuscol_click_defs array
 
-  // Window bar click definitions
-  StlClickDefinition *w_winbar_click_defs;
-  // Size of the w_winbar_click_defs array
-  size_t w_winbar_click_defs_size;
-
-  // Status column click definitions
-  StlClickDefinition *w_statuscol_click_defs;
-  // Size of the w_statuscol_click_defs array
-  size_t w_statuscol_click_defs_size;
+  buf_T *w_conceal_line_buf;                    // buffer in win when first invoked
+  bool w_conceal_line_provider;                 // whether conceal_line provider is active
 };