Add configurable keybindings for line-by-line scrollback scrolling

[nogu3]

Problem

Plain PageUp/PageDown scroll the focused pane's scrollback by a full page, and copy mode offers line-precise movement, but there is no way to scroll the scrollback line-by-line without entering copy mode.

Coming from terminal multiplexer setups where e.g. shift+PageUp/PageDown scroll by a single line, this is a habit that's hard to replace — entering copy mode just to nudge the view a couple of lines feels heavyweight.

Proposal

Would you consider either of these?

• Optional [keys] actions like scroll_line_up / scroll_line_down (unset by default), or
• Treating shift+PageUp/PageDown as 1-line scroll in the existing page-key interception.

Currently modified page keys are intentionally forwarded to the pane app (src/app/input/terminal.rs, "Modified page keys are pane shortcuts"), so an opt-in binding would not break existing behavior.

Environment

herdr 0.7.3 (stable), Linux (WSL2) and macOS

(Originally filed as #1283; moved here per the issue policy.)