Add a variant of `ListFormat#formatToParts` that returns the source objects

[dead-claudia]

This would be immensely useful for things like virtual DOM lists and polyfilling whatwg/html#4630 without a ton of pain.

I'm thinking of an API like this:

interface ListFormat {
    // Existing
    formatToParts(list: string[]): ListFormatPart<string>
    // New
    formatToElements<T>(list: Array<{value: T, part: string}>): ListFormatPart<T>
}

type ListFormatPart<T> =
    | {type: "element", value: T}
    | {type: "literal", value: string}

The key functional difference is that formatToElements, while it coerces its arguments, it does not return the coerced results.

Polyfilling this is very awkward and only works because Unicode explicitly states that previous parts must come before next parts:

Intl.ListFormat.prototype.formatToElements = function (list) {
    const parts = []
    const values = []

    for (const {part, value} of list) {
        parts.push(part)
        values.push(value)
    }

    const result = this.formatToParts(parts)

    let index = 0
    for (const part of result) {
        if (part.type === "element") {
            part.value = values[index++]
        }
    }

    return result
}