slint-compiler: Guess output format from file extension if not present

This is more intuitive that the C++ default, especially in light of additional output languages in the future.

To be on the safe side, this also passes the explicit -f cpp on the CMake side.

api/cpp/cmake/SlintMacro.cmake

@@ -73,6 +73,7 @@ function(SLINT_TARGET_SOURCES target)
         add_custom_command(
             OUTPUT ${CMAKE_CURRENT_BINARY_DIR}/${_SLINT_BASE_NAME}.h ${cpp_files}
             COMMAND ${SLINT_COMPILER_ENV} $<TARGET_FILE:Slint::slint-compiler> ${_SLINT_ABSOLUTE}
+                -f cpp
                 -o ${CMAKE_CURRENT_BINARY_DIR}/${_SLINT_BASE_NAME}.h
                 --depfile ${CMAKE_CURRENT_BINARY_DIR}/${_SLINT_BASE_NAME}.d
                 --style ${_SLINT_STYLE}