Pick `bufnr` function, to allow selecting which split to open a selected file in

[Leandros]

• vim or neovim?
  • [ ] vim
  • [x] neovim

• Output of vim --version or nvim --version:

  
  NVIM v0.9.2
  Build type: Release
  LuaJIT 2.1.1692716794
  
  system vimrc file: "$VIM/sysinit.vim"
  fall-back for $VIM: "/usr/local/share/nvim"

Run :checkhealth for more info

- Output of `:echo has("python")`: `0`
- Output of `:echo has("python3")`: `1`
- Output of `:py print(sys.version)`: 

3.7.16 (default, Aug 30 2023, 20:37:53) [GCC 7.3.1 20180712 (Red Hat 7.3.1-15)]

- Output of `:py3 print(sys.version)`: 

3.7.16 (default, Aug 30 2023, 20:37:53) [GCC 7.3.1 20180712 (Red Hat 7.3.1-15)]

- Operating system: <!-- use [x] to select, note: no space in between brackets -->
    - [x] Linux
    - [x] Mac OS X
    - [ ] Windows
    - [ ] Etc.
- Configurations related to LeaderF in vimrc: 

### Describe your question, feature request, or bug.

LeaderF will always open the specified file in the least recently focused buffer (the buffer that you previously edited before triggering leaderf). 

I would like to be able to select which buffer the file is opened in, similar to what I can achieve with `nvim-tree` and `window-picker`:
<img width="961" alt="Screenshot 2023-09-15 at 11 58 33" src="https://github.com/Yggdroot/LeaderF/assets/396475/7e1b3c74-f957-441e-9059-2fd8eedd982b">

The green bars below each split is telling me which key to press to open the file in this split.

As far as I understand, `LeaderF` would need to provide a "callback" that's called before opening the file. The `pick()` function from `window-picker` will return a bufnr in which the new file is opened.

[Leandros]

There is also vim-choosewin: https://github.com/t9md/vim-choosewin

[Yggdroot]

Why not move the cursor to the destination window, then use LeaderF?