Refactoring to use more builtins (display-buffer)

[gagbo]

Hello,

Thanks for the package. When I started using it I noticed that the windows from the vterm buffers didn't respect the customizations that I wanted to put in display-buffer-alist, so I made a few changes. I don't know if you will want to merge them (there are a lot), and I didn't test completely the changes:

• Use pop-to-buffer instead of manual window manipulations and switch
• Change the defcustom to mimic parameters from display-buffer-alist to control window positions
• Simplify the toggle functions to use more extensively get-buffer and get-buffer-window
• Use the -- convention for internal functions
• Rename multi-vterm-get-buffer to show that it creates a buffer on miss, just like the difference between get-buffer and get-buffer-create

There are probably a few other changes. I set the PR as draft because I would like to test those changes on my setup for a few weeks before, but I'll take all feedback (even if it is "I won't merge it")