Separate `StreamLineBuffer` from `ConsoleBuffer`

[Vankata453]

Moves the line buffering functionality of ConsoleBuffer to a base class.

This simplifies the code a little bit, as well as allows using the line buffering functionality of ConsoleBuffer separately, if needed.

(I thought I'd need this to implement a feature, however, turns out I don't. It could still be implemented as a code improvement, though.)