irremote: add a NEC transmitter - #898

Thanks for this. Looks good overall. A few small things:

1. The stop channel is closed but nothing ever receives from it. It only works as a token for the repeat goroutine to see it was replaced. A simple counter would do the same job and be easier to follow.

2. wait() busy loops on time.Now(), so timing accuracy depends on the clock resolution. On SAMD51 that is about 122µs (see irremote unreliable on SAMD51 #668), which is a big error on a 562.5µs mark. Fine on ESP32 and RP2040, but worth a short note in the docs.

3. The API is a little uneven. SendNEC takes a byte command while the receiver Data.Command is uint16. Also SendNECRawCode and SendNECRepeat are exported and return a time.Duration that callers probably do not need.

4. The extra NECZeroSpace after the trailing mark is not needed, since the carrier is already off at that point. It just adds 562.5µs to each frame.