added I2C TwoWire::busIdle to be checked by writeReadAsync

[veecle-stefan]

Problem
When using writeReadAsync in a multi-threaded environment (e.g. with FreeRTOS), the DMA interrupt fires immediately after the TX FIFO is loaded—not when the I2C peripheral has actually finished clocking out those bytes. As a result, a new writeReadAsync call might start too early and interrupt the ongoing hardware transfer. This issue only affects write-only transactions (rbytes == 0), because read transactions already use a different interrupt that waits for RX completion.

Solution
This PR adds a bool busIdle() function that checks if the I2C hardware is truly idle (i.e., no master activity and the transmit FIFO is empty). Inside writeReadAsync, if the bus is still busy, we return false—just as we already do if a DMA transfer is in progress. This allows the caller to detect the “bus still busy” case and retry the operation once the bus becomes free.

Benefits

• Prevents partial writes when a new transaction starts too soon.
• Allows cleaner multi-thread or asynchronous usage of writeReadAsync by giving callers a straightforward way to avoid interrupting a still-active write.
• In testing, this improved stability and reduced CPU load significantly when accessing complex sensors (e.g. Adafruit BNO085) at higher rates.

[earlephilhower]

Nice addition, looks good to me!