feat: Better border joining

[allyring]

I think it'd be nice to have text that's been rendered with borders to be able to join without gaps.

I think the main problem to solve here would be detecting corners and correctly replacing them with different box-joining characters (e.g.: ┤, ┼, etc), because the majority of the shared borders are usually the same character (e.g.: ─, │)

A manually done example:

           ││           
           ││           
box 1      ││      box 2
           ││           
           ││           
───────────┘└───────────
────────────────────────
box 3

to

           │           
           │           
box 1      │      box 2
           │           
           │           
───────────┴───────────
box 3

I don't know if this would really allow for more features to be developed that take advantage of this or not, but it looks a lot nicer for more complex applications in my opinion.