Add support for "surrogate pairs" in unmatched characters

[webpro]

Thanks for a great package, been very useful.

Hit an odd bug:

const input = "██ █";
const result = getStringTruncatedWidth(input, { limit: 2 });
// { width: 2, index: 4, truncated: true, ellipsed: true }

The index should be 2. While this looks fine:

const input = "█ ██";
const result = getStringTruncatedWidth(input, { limit: 2 });
// { width: 2, index: 2, truncated: true, ellipsed: true }

Also, not sure if you even want to support this, feel free to reject. But I've tried to fix up the unmatched part anyway. Also not sure if it's up to your vision/standards, but happy to adjust.