vslot not respected in popup rules

### I confirm that...

- [x] I searched the issue tracker and found no similar issues.

- [x] I searched the [documentation](https://docs.doomemacs.org), [FAQ](https://docs.doomemacs.org/-/faq), [discussion board](https://doomemacs.org/discuss), and [Google](https://google.com) for solutions to my issue.

- [x] I read [the debugging guide](https://doomemacs.org/d/how2debug) and researched the issue to the best of my ability.

- [x] My issue can be observed on the latest available commit of Doom.

- [x] My issue can be observed on a stable release of Emacs (i.e. doesn't end in `.50`, `.60`, or `.90`–`.99`)

### Describe your issue

When using `:side 'right`, no matter what values i supply for `:vslot`, popups stack in order of appearance from left to right.

### Steps to reproduce

1. Set up 2 popup rules using `set-popup-rule!`, with `:side 'right`, a positive vslot value for popup 1, e.g. `10` and a negative vslot value for popup 2, e.g. `-10`
```elisp
(set-popup-rule! "^\\*popup1"
:side 'right
:vslot 10)

(set-popup-rule! "^\\*popup2"
:side 'right
:vslot -10)
```
2. Observe opening/closing popups
```elisp
(pop-to-buffer (get-buffer-create "*popup1*")) ;; opens on the right 👍
(pop-to-buffer (get-buffer-create "*popup2*")) ;; opens to the right of popup1 👍
(delete-window (get-buffer-window "*popup1*"))
(pop-to-buffer (get-buffer-create "*popup1*")) ;; opens to the right of popup2 👎
```

### System information

https://pastebin.com/22PXmPzH