Override this to provide a different tab competition starting point. The default
is 0, always completing the complete line, but you may return the index of another
character of candidate to provide a new split.
Both candidate and afterCursor may have private data packed into the dchar bits
if you enabled enableAutoCloseBrackets. Use ch & ~PRIVATE_BITS_MASK to get standard dchars.
Override this to provide a different tab competition starting point. The default is 0, always completing the complete line, but you may return the index of another character of candidate to provide a new split.