feat: add a tool that can fix line joints

[widcardw]

This pr is to introduce the tool that can turn

into

We can use it just like the eraser.

The features are listed below

──x──  into ─────

  │           │  
  │           │  
  x    into   │
  │           │  
  │           │  

 x───  into   ┌─────
 │            │     

──x──  into  ──┬──
  │            │  

  │           │  
  │           │  
──x──  into ──┼──
  │           │  
  │           │  

I think this tool is useful, but my code is a little strange. Please review and give me any suggestion. Thanks!

[huytd]

Hey, sorry for late response, and thank you so much for contributing this feature. I need more time to check this, and will get back to you in a few days :D

[widcardw]

Oops, I mistakenly closed this pr... You can reopen it any time. Thanks!