feat: add borderstyle option

[CatsDeservePets]

• Fixes [Feature request] Equivalent of Ranger's set draw_borders separators setting #1438
• Supersedes feat: teach drawbox additional options #1439

This PR adds a new option borderstyle. It extends the capabilities of lf's drawbox option to allow for ranger like border designs.

The following values are allowed: box (default), roundbox (replaces set roundbox), round, outline, roundoutline and separators.

[nodrawbox]	separators
box	outline
roundbox	roundoutline

Previous Description

This PR is essentially a 1:1 implementation of ranger's draw_borders option and should not be considered finale.

From ranger's man page:

draw_borders [string]
Draw borders around or between the columns? Possible values are:

none           no borders of any sort
outline        draw an outline around all the columns
separators     draw only vertical lines between columns
both           both of the above

set draw_borders none equals set drawbox false/set nodrawbox
set draw_borders both equals set drawbox true/set drawbox
Note: set draw_borders equals set drawbox false/set nodrawbox (ranger interprets it as an empty string which is equal to none)

Due to the existing drawbox option being of type bool, extending it while maintaining backward compatibility would be awkward.

Therefore, it was suggested to deprecate it in favour of a new enum option. However, this goes against lfs philosophy and I consider drawbox being a core feature of lf.

none	outline

separators	both

[CatsDeservePets]

Hello @joelim-work! What are your thoughts on this?

[joelim-work]

My main concern with this exact implementation is that removing drawbox is a breaking change, and I expect there to be many users as well. Now you can no longer toggle borders using something like set drawbox!. I am probably fine with another enum string option like borderstyle or similar that takes effect only if drawbox is enabled, and perhaps it could be merged with roundbox too.

[joelim-work]

LGTM, I had a couple of very minor comments but they are very trivial. Free free to merge.