Open external editor at the selected diff line

[davxy]

This Pull Request fixes/closes #1206.

It changes the following:

• open the external editor at the line selected in the diff view (vim, nvim, nano, emacs, helix)
• For an editor that is not in the list, gitui runs the same command as before

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog