Add option for Helix `xc` behaviour

[jgdhs27]

In Helix, when you use c with a full line selected including the newline, the newline character will not be deleted. This behavior is most noticeable when you do xc. In Helix, this will clear the line but not the newline, keeping an empty line.

Kakoune and Dance treat this differently - xc will delete the line including the newline character.

Because this behaves differently in Kakoune and Helix, I believe this behavior should be an option with different defaults in Helix and Kakoune.