RP2350 (Pico 2) support and fix BPM-too-slow problem on RP2040/RP2350

[doctea]

Very simple patch, its the same as for RP2040, just needed to update the #ifdef check to check for ARDUINO_ARCH_RP2350 in addition to ARDUINO_ARCH_RP2040 :)

[doctea]

This also now solves long-standing problem with BPM being a little too slow. Should now be perfectly accurate!

Problem was needing to cast interval from uint32_t to int32_t so that the correct negative interval is passed to add_repeating_timer_us (because "the repeat delay in microseconds; if >0 then this is the delay between one callback ending and the next starting; if <0 then this is the negative of the time between the starts of the callbacks.")