feature: upstream capture_since #740

The https://github.com/tmux-python/libtmux-mcp/pull/63/changes#top implements capture_since, which would be a fantastic addition to this library alongside capture_pane. In particular, it should just involve implementing a cursor class, which notes internally the pane it is associated to. Then the function capture_since, which returns the text from the given cursor to the end, along with a new cursor representing the new end. capture_since could even be a method of that cursor class, but I'd push for a functional interface, where calling it doesn't modify the cursor.