Use Callbacks intead of buffer for output

Currently, the output from libvterm is implemented using a (fixed-size) byte buffer. This has the potential to fill up, due to the fact that the embedding application might not be reading it fast enough, or that the input contains "packet amplifiers" like DECRQSS or similar.

It would be more robust if instead, the application provided a write-like callback function so it can perform its own buffer management, or whatever is required, instead of libvterm having to buffer it itself.

There might need to be special consideration of a special error value to mean "retry again"; asking libvterm to not consume the final byte of input that caused it to need to emit this string, and instead do it again later once the application has been given a chance to rearrange. However, that's not going to be possible for input events, so it remains to be seen how useful this could be.