feat(ui): emit "msg_clear" event after clearing the screen (#34035)

Problem:  ext_messages cannot tell when the screen was cleared, which is
          needed to clear visible messages. An empty message is also
          never emitted, but clears messages from the message grid.
Solution: Repurpose the "msg_clear" event to be emitted when the screen
          was cleared. Emit an empty message with the `empty` kind to
          hint to a UI to clear the cmdline area.

test/functional/lua/ui_event_spec.lua

@@ -196,7 +196,7 @@ describe('vim.ui_attach', function()
         pos = 0,
       } },
     })
-    feed('version<CR><CR>v<Esc>')
+    feed('version<CR>')
     screen:expect({
       grid = [[
         ^2                                       |
@@ -208,7 +208,7 @@ describe('vim.ui_attach', function()
         screen.messages = {} -- Ignore the build dependent :version content
       end,
     })
-    feed([[:call confirm("Save changes?", "&Yes\n&No\n&Cancel")<CR>]])
+    feed([[v<Esc>:call confirm("Save changes?", "&Yes\n&No\n&Cancel")<CR>]])
     screen:expect({
       grid = [[
         ^4                                       |