There is no single SQL * Plus command for this, but you can create one script that calls all the others:
Put the following in a batch file
@echo off
echo.>"%~dp0all.sql"
for %%i in ("%~dp0"*.sql) do echo @"%%~fi" >> "%~dp0all.sql"
, script all.sql , . .sql , .
sqlplus user/pwd @all.sql ( sqlplus all.sql script)