filename = love.filesystem.getUserDirectory()..'/lines.txt',  -- '/' should work even on Windows

    filename = love.filesystem.getSourceBaseDirectory()..'/lines.txt',  -- '/' should work even on Windows

By default, lines.love reads/writes the file `lines.txt` in your default
user/home directory (`https://love2d.org/wiki/love.filesystem.getUserDirectory`).

By default, lines.love reads/writes the file `lines.txt` in a directory
specific to this app (https://love2d.org/wiki/love.filesystem.getSourceBaseDirectory).