Allow very large scrollback

[junkblocker]

Is your feature request related to a problem? Please describe.

I am surprised to find that wezterm holds the whole scrollback in memory and doesn't attempt to swap out to disk or even just use a circular buffer or something even in memory. This causes wezterm to run out of memory and get killed.

Describe the solution you'd like

Swap lines beyond the scrollback_lines out to disk still keeping them available via pane:get_lines_as_text(...) .

Describe alternatives you've considered

Reduce the scrollback_lines size which is inadequate since that then requires changing your tools/habits.

Additional context

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