feat(ext_messages): add hl_id to ext_messages chunks

Problem:  Ext_messages chunks only contain the highlight attr id, which
          is not very useful for vim.ui_attach() consumers.
Solotion: Add highlight group id to message chunks, which can easily be
          used to highlight text in the TUI through nvim_buf_set_extmark():
          hl_group = synIDattr(id, "name").

test/functional/lua/ui_event_spec.lua

@@ -151,9 +151,9 @@ describe('vim.ui_attach', function()
       {
         'msg_history_show',
         {
-          { 'echomsg', { { 0, 'message1' } } },
-          { '', { { 0, 'message2' } } },
-          { 'echomsg', { { 0, 'message3' } } },
+          { 'echomsg', { { 0, 'message1', 0 } } },
+          { '', { { 0, 'message2', 0 } } },
+          { 'echomsg', { { 0, 'message3', 0 } } },
         },
       },
     }, actual, vim.inspect(actual))