Custom classes not being merged correctly in `tailwind-merge`

Thank you @BIA3IA for pointing this out!

As mentioned in my review of #26, we should extend the `twMerge` function in `@/lib/utils.ts` to ensure correct merging of `tailwind` classes.
There I proposed an easy solution regarding the border radii, but there might / might not be the same problem with colors/typography/shadows/etc. (needs testing, but it probably affects everything).