bxs: outline should support inset, and suggestion to add bxs:stroke (inset) for 1px shadows

Currently outline doesn't support inset from my experience.
It'd be great if it did.
```imba
css bxs: outline hue2 # >>> bxs:0 0 0 3px hue2
css bxs: outline hue2 inset # >>> bxs:0 0 0 3px hue2 inset
```

I also often use 1px box shadows in UI, so this would be handy as well.
```imba
css bxs: stroke hue1 # >>> bxs:0 0 0 1px hue1
css bxs: stroke hue1 inset # >>> bxs:0 0 0 1px hue1 inset
```
alternatively add a pixel argument to override outline width. it'd save few characters, but it would at least be more legible.
```imba
css bxs: outline hue2 1px # >>> bxs:0 0 0 1px hue2
css bxs: outline hue2 1px inset # >>> bxs:0 0 0 1px hue2 inset
```

Start from the implementation of the bxs: outline syntax; no file or test is named in the issue. Compare the requested inset and 1px stroke or width forms with the shown box-shadow outputs, and consider which alternative is intended before defining done.

Written by the indexing model from the issue text.