Update vim config for markdown and vimwiki surround

- Define surround mapping for bold text (**) in markdown and vimwiki

home/common/core/nixvim/default.nix

@@ -60,6 +60,12 @@
       let g:db_ui_hide_schemas = ['pg_catalog', 'pg_toast_temp.*', 'pg_toast']
       let g:db_ui_use_nerd_fonts = 1
       let g:db_ui_execute_on_save = 0
+
+      " == custom surround
+      augroup initvim
+        au!
+        autocmd FileType markdown,vimwiki let b:surround_{char2nr('b')} = "**\r**"
+      augroup END
     '';
   };
 }