feat: add register / which stores the parent path of currently opened document

[nikitarevenco]

This register is going to be extremely useful for the following operations:

• Creating a file next to the currently edited file with :o <C-r>/hello.txt
• Changing directory to the directory of the currently edited file with :cd <C-r>/
• Changing directory into parent directory or traveling across the file system in an easier manner. One could :pwd then cd .., but with this you can easily see where you are going and manipulate your path

Inspired by this discussion, where it seems that this feature would really be beneficial: #12002

[darshanCommits]

I think this is better handled by #11164 (command expansion) PR.

[nikitarevenco]

I think this is better handled by #11164 (command expansion) PR.

I can totally see that! It will be awesome when that gets merged