Implement receiver_buffer_size kwarg

Implement `receiver_buffer_size` kwarg to match `busio`:
https://docs.circuitpython.org/en/latest/shared-bindings/busio/index.html#busio.UART

Ideally using background read so the library doesn't block when awaiting (larger) chunks of data.

Motivation: PIO FIFOs are very limited in size. Lower baudrates and / or hardware flow control can help in some use cases.

Start with the repository's existing UART API and compare it with the linked busio.UART documentation, especially receiver_buffer_size behavior. Determine how reads currently interact with the PIO FIFO before deciding how background reads should work. Done means the kwarg matches busio and awaiting larger chunks does not block, with behavior suitable for the limited PIO FIFO.

Written by the indexing model from the issue text.