Allow multi-character corners on borders

[Grantmartin2002]

Is your feature request related to a problem? Please describe.
I am trying to make borders with the following style:

  ┌──                                             ── ┐                                                                                                        
  │                                                  │                                                    
                                                                                               
                                                                                                                                                                              
  │                                                  │                                                                                                    
  └──                                              ──┘    

Describe the solution you'd like
Multi-character corner support for lipgloss.Border (it currently truncates corners to 1 rune)

Describe alternatives you've considered
Manual border rendering

Additional context