feat: Add support for custom `set_lines` function

[artrixdotdev]

Goal

This PR lets users provide their own set_lines function through config.opts.

Why?

This makes it possible to work with plugins like
baleia.nvim, and allows things like
ANSI-colored ASCII art or logs to be shown on startup.

Notes

• No breaking changes
• Core functionality remains the same if no custom set_lines is provided