Add `list-style-image` utility

[neupauer]

This PR will add the list-style-image utilities.

This PR also fix the ambiguity of the list utility by introducing the string: type for the arbitrary values, so: .list-['👉'] will generatelist-style-type: '👉' .list-[url(...)] will generate list-style-image: url(...)

💥This also introduces breaking change to the list-style-type utility:

- list-[var(--value)]
+ list-[string:var(--value)]

[adamwathan]

Thanks! I think to avoid the breaking change we are going to want to come up with some clever internal way to make list-[var(--value)] still work, maybe like a preferred: true or preferred: somePrivateSymbol option or something.

I think this would be good to add though so going to leave this open so we can look into it soon — just going through PRs today as we're trying to clean up before tagging v3.1. Will try to dig into this more for v3.2 👍🏻

[thecrypticace]

I updated the PR to use list-style-type for otherwise unknown types (the any type) which fixes the issue with var(…) and the need for the additional string data type — which I have removed it as well.

ping @adamwathan