Supports a syntax like this

(#insertion-delimiter! @collectionItem "@collectionItem.trailing" "@collectionItem.leading" ", ")

After I implemented it I do wonder if we want something simpler? Maybe a boolean just telling it to include delimiters?

(#insertion-delimiter! ", " true)

Edit: I have another approach in the below pull request that I prefer. I will keep this draft here as an alternative to discuss.
#2081

@pokey @josharian

Checklist

• I have added tests
• I have updated the docs and cheatsheet
• [-] I have not broken the cheatsheet