Add rp2040 task queue support for Core #1

[mikee47]

This PR re-implements the task queue using SDK-provided queue API. This allows code running on Core #1 to queue tasks for execution by Sming (which runs on Core #0).

Some care is required running both cores together so some notes have been added to the documentation.

Note: I had a need to run some simple background analogue processing for which core 1 was an ideal solution.
I put all the code in RAM, both for performance reasons and because XIP is suspended during flash erase/write operations (such as file writes). I avoided using floating-point code for the same reasons - FP routines are large so compile to flash.

The Cortex M0+ has hardware support for integer division but these routines compile to flash by default. They're small so have set the SDK PICO_DIVIDER_IN_RAM flag. It seems reasonable that integer arithmetic shouldn't require flash accesses.

Note: There is also PICO_INT64_OPS_IN_RAM and PICO_MEM_IN_RAM which probably warrant further investigation/consideration.

[what-the-diff]

PR Summary

• Added new pin A0
  A new pin A0 has been introduced for further functionality.

• Moved queue_add_internal function to RAM
  The queue_add_internal function has been relocated to RAM, which resolves issues related to interrupts and function calls.

• Ensured system tasks run properly after boot
  Fixed an issue where system tasks wouldn't run if queued before user task queues were created. A static TaskQueue object now handles system tasks, guaranteeing the functionality even when no user-defined task queues exist yet.