Flag emoji icons for quick bar and text selection popover don't work

[twschiller]

Describe the bug

Emoji icons for quick bar and text selection popover don't work for the flag emoji

To Reproduce

Create a Quick Bar or Context Menu with 🇺🇸 at the beginning

Actual behavior

Not detected as an emoji, so PixieBrix displays the default box icon

Expected behavior

Emoji shows as the icon in the Quick Bar and Text Selection Popover

Screenshots/Loom

https://files.slack.com/files-pri/T017AEVUURF-F06P0GGULDT/image.png

Desktop (please complete the following information):

• Main

Related Code

• Likely a bug in the splitStartingEmoji method: https://github.com/pixiebrix/pixiebrix-extension/blob/c5639219f59a52fe0970ae120c9e9531f576bb77/src/utils/stringUtils.ts#L24-L24