Future feature: use slash-modifier syntax for border-width if and when it becomes fully supported for matchUtilities

[MichaelAllenWarner]

At the moment, the ability to use slash-modifier syntax for matchUtilities() is behind an experimental features flag: tailwindlabs/tailwindcss#9541

If and when this becomes non-experimental, perhaps refactor this plugin to use it for border-width, instead of having to do the grid-inner-[n,m] thing.