Format a list in English

Implement listFormat(items, options). It turns an array of strings into a readable English list, the kind you see in "Ada, Linus and 2 others liked this".

Basic format:

• no items: ""
• one item: the item itself, "Ada"
• two items: "Ada and Linus"
• three or more: commas between items and and before the last, with no comma before and: "Ada, Linus and Grace"

Options are all optional. First drop empty strings, then apply the options in this order:

1. unique: true removes repeats, keeping each item's first occurrence.
2. sorted: true sorts the items in ascending order with the default sort(), which compares UTF-16 code units.
3. length: n shows only the first n items, then and K others, where K is how many were left out. Use other when K is 1. If n is not a positive number smaller than the item count, ignore it.

Don't modify items, and don't use Intl.ListFormat.