Serial send and receive buffer size. The receive buffer is often used as another streaming
// buffer to store incoming blocks to be processed by Grbl when its ready. Most streaming
// interfaces will character count and track each block send to each block response. So,
// increase the receive buffer if a deeper receive buffer is needed for streaming and avaiable
// memory allows. The send buffer primarily handles messages in Grbl. Only increase if large
// messages are sent and Grbl begins to stall, waiting to send the rest of the message.