feat(ui): avoid setting 'cmdheight' with vim.ui_attach()

Problem:  We allow setting 'cmdheight' to 0 with ext_messages enabled
          since b72931e7. Enabling ext_messages with vim.ui_attach()
          implicitly sets 'cmdheight' to 0 for BWC. When non-zero
          'cmdheight' is wanted, this behavior make it unnecessarily
          hard to keep track of the user configured value.
Solution: Add set_cmdheight to vim.ui_attach() opts table that can be
          set to false to avoid setting 'cmdheight' to 0.

src/nvim/lua/executor.c

@@ -682,7 +682,7 @@ static int nlua_ui_attach(lua_State *lstate)
     return luaL_error(lstate, "invalid ns_id");
   }
   if (!lua_istable(lstate, 2)) {
-    return luaL_error(lstate, "ext_widgets must be a table");
+    return luaL_error(lstate, "opts must be a table");
   }
   if (!lua_isfunction(lstate, 3)) {
     return luaL_error(lstate, "callback must be a Lua function");
@@ -699,13 +699,18 @@ static int nlua_ui_attach(lua_State *lstate)
     const char *s = lua_tolstring(lstate, -2, &len);
     bool val = lua_toboolean(lstate, -1);
 
-    for (size_t i = 0; i < kUIGlobalCount; i++) {
-      if (strequal(s, ui_ext_names[i])) {
-        if (val) {
-          tbl_has_true_val = true;
+    if (strequal(s, "set_cmdheight")) {
+      ui_refresh_cmdheight = val;
+      goto ok;
+    } else {
+      for (size_t i = 0; i < kUIGlobalCount; i++) {
+        if (strequal(s, ui_ext_names[i])) {
+          if (val) {
+            tbl_has_true_val = true;
+          }
+          ext_widgets[i] = val;
+          goto ok;
         }
-        ext_widgets[i] = val;
-        goto ok;
       }
     }
 
@@ -715,11 +720,12 @@ ok:
   }
 
   if (!tbl_has_true_val) {
-    return luaL_error(lstate, "ext_widgets table must contain at least one 'true' value");
+    return luaL_error(lstate, "opts table must contain at least one 'true' ext_widget");
   }
 
   LuaRef ui_event_cb = nlua_ref_global(lstate, 3);
   ui_add_cb(ns_id, ui_event_cb, ext_widgets);
+  ui_refresh_cmdheight = true;
   return 0;
 }