api/buffer: add "on_bytes" callback to nvim_buf_attach

This implements byte-resolution updates of buffer changes.
Note: there is no promise that the buffer state is valid inside
the callback!

src/nvim/fileio.c

@@ -1797,6 +1797,7 @@ failed:
       linecnt--;
     }
     curbuf->deleted_bytes = 0;
+    curbuf->deleted_bytes2 = 0;
     curbuf->deleted_codepoints = 0;
     curbuf->deleted_codeunits = 0;
     linecnt = curbuf->b_ml.ml_line_count - linecnt;