How to Add Icon to Checkout UI Extensions

I’d like to add a new Icon to the list of available icons.

Specifically the ShieldCheckMarkIcon from Polaris.

Happy to contribute, just not sure where to put the PR. I found the types of icons supported, just not sure if and where I can actually add the missing icon. Thank you.

Custom icon support as well: [UI Extensions] Custom SVG source for Icon · Issue #1310 · Shopify/ui-extensions · GitHub