Add % current file register

[pickfire]

Useful to open files relative to current file.

The experience is worse especially on files that have huge paths, like python virtual environment, this allows browsing relative files easier, while being a lot easier to implement compared to find file. #2412

To create new file from current file directory.

:o <C-r>%<C-w>new-file<ret>

Compared to previous method (mouse copy-paste or manually type - and I usually manually type 1-7 directories)

:o <mouse select file name><middle click paste><manually type full path if truncated><C-w>new-file<ret>

[archseer]

Duplicate of #3008 was replaced by #3393

[pickfire]

I think #3393 is slightly different and a lot harder to use compared to this scenario, the user has to type %val{dirname} (16 keystrokes) as compared to <C-r>%<C-w> (6 keystrokes).

The one implemented here is slightly similar to what @QiBaobin implemented, except being pull based (when user select registers) rather than push based (when user change view). Now I just realized I should clear the register if user did :new when I saw vim shows an error when there is no %, I think he didn't implement that too.

[the-mikedavis]

The register could be useful for prompts and pickers where variable substitution might not be done (it might not make sense to replace variables if the prompt is a regex for example).

Kakoune has a special register for this as well: https://github.com/mawww/kakoune/blob/master/doc/pages/registers.asciidoc#special-registers

[the-mikedavis]

Kakoune has an interesting way of passing state like the current buffer name to register functions: registers are defined with an anonymous function that takes Context and returns the value of the register. This is helpful for other special registers like 0..9 acting as regex captures. We don't need to emulate that exactly but I'm curious if we can implement this in a way that's easier to add the other special registers later

[pickfire]

@the-mikedavis I did what you mentioned but now I don't like the API to always return an owned value, we need to keep cloning since the readonly function should return an owned value, unless we get it to return &str which will limit what we can do in the future.