Telecommand: `freertos_demo_stack_usage`, which allocates a variable-length array on the stack to test the FreeRTOS stack usage

[DeflateAwning]

Create a new telecommand named freertos_demo_stack_usage.

It should accept 1 arg: the number of bytes to allocate.

This is for testing the freetos_list_tasks_jsonl command, and to ensure we fully understand what happens when we run out of stack space.