feat: teach `drawbox` additional options

[wincent]

Follows the pattern established by Ranger's draw_borders setting:

• true / both: draw an outline box around all panes, plus vertical separators between panes.
• outline: draw only the outline box.
• separators: draw only the separators.
• false / none: draw neither outline box nor separators (default).

So, this is a string option that accepts "true" and "false" strings for backwards compatibility.

Note the behavior of toggling with :set drawbox!: if any borders are being drawn (ie. "both", "outline" or "separators", turns them off; if no borders are being drawn (ie. "none"), turns them all on (ie. "both").

Closes: https://github.com/gokcehan/lf/issues/1438

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Context: I'm not a Go expert and I'm new to this codebase, so this is my first PR, so any and all feedback is welcome.