strsplit_ctl with control character splits

[brodieG]

This should probably work:

fansi::strsplit_ctl("hello\nworld", "\n")
# [[1]]
# [1] "hello\nworld"

At a minimum, should be clearly documented. Need to think about what it means if the splitting element is actually an SGR that would change the display state. Is the state changed, or not because the split just obliterates the SGR?