The Arduino IDE and CLI now have support for the Pico and the Nano RP2040 Connect! This was developed by the Arduino team and is available for free for anyone.
Just open the Board Manager dialog and look for their names: you’ll find the two new cores. If you have a suitable board, do help us testing them! You can report any feedback in this thread. Thank you!
If you want to contribute to development, the source code is located in this repository: GitHub - arduino/ArduinoCore-mbed