Feat: Make copy paste friendly

[tnwei]

Resolves #2 by changing the ascii box drawn around each code cell to just a single line header.

Before:

    In [1]:                                                              
    ╭───────────────────────────────────────────────────────────────╮    
    │ # Simple print output                                         │    
    │ print("Hello from nbread test!")                              │    
    │ print("Testing print output")                                 │    
    ╰───────────────────────────────────────────────────────────────╯    
    Hello from nbread test!                                              
    Testing print output                                                 
    In [2]:                                                              
    ╭───────────────────────────────────────────────────────────────╮    
    │ # Execute result (return value)                               │    
    │ x = 40 + 2                                                    │    
    │ x                                                             │    
    ╰───────────────────────────────────────────────────────────────╯    
    Out[2]:                                                              
    42                                                                                                            

After:

    ── Code [1] ──────────────────────
    # Simple print output                                                                                                                                                    
    print("Hello from nbread test!")                                                                                                                                         
    print("Testing print output")                                                                                                                                            
                                                                                                                                                                             
    Out[1]:                                                                                                                                                                  
    Hello from nbread test!                                                                                                                                                  
    Testing print output                                                                                                                                                     
                                                                                                                                                                             
    ── Code [2] ──────────────────────
    # Execute result (return value)                                                                                                                                          
    x = 40 + 2                                                                                                                                                               
    x                                                                                                                                                                        
                                                                                                                                                                             
    Out[2]:                                                                                                                                                                  
    42                                                                                                                                                                       
                   

Less aesthetic, but more practical. I mean, this sentiment held true since I opened the issue two years ago. Sorry I just got around to scratch this itch