fix(messages)!: vim.ui_attach message callbacks are unsafe

Problem:  Lua callbacks for "msg_show" events with vim.ui_attach() are
          executed when it is not safe.
Solution: Disallow non-fast API calls for "msg_show" event callbacks.
          Automatically detach callback after excessive errors.
          Make sure fast APIs do not modify Nvim state.

src/nvim/generators/gen_api_dispatch.lua

@@ -750,7 +750,7 @@ local function process_function(fn)
     write_shifted_output(
       [[
     if (!nlua_is_deferred_safe()) {
-      return luaL_error(lstate, e_luv_api_disabled, "%s");
+      return luaL_error(lstate, e_fast_api_disabled, "%s");
     }
     ]],
       fn.name