Development tools for Zephyr on STM32Workbench for Zephyr: VS Code extension to set up, build, flash and debug Zephyr applications on STM32.Devicetree manager: browse, edit and visualize Zephyr devicetree, STM32CubeMX-like for Zephyr.
The STM32F103 is supported on System Workbench but was only released quite recently; could you update your IDE (Help >> Check for Updates...) and test again?