Add Mbed CE support for STM32 parts

[multiplemonomials]

This PR adds support for using Mbed CE with STM32 chips. See the forum thread for more information: https://community.platformio.org/t/hi-from-mbed-ce-project/48249

Note that to make this MR, I went through all of the existing boards that declared mbed support, checked them against the Mbed CE target list, and verified that they are supported before adding them. I also found a few boards that didn't have mbed support but should have mbed-ce support, including Arduino Portenta/Giga (not supported in ARM Mbed, we added these ourselves), and nucleo_g031k8 (ARM Mbed did actually support this one).

Also note that for reviewing this PR, the mbed ce target list webpage might be useful!