chore: add XDC native asset as erc20:0x0 stopgap

xdc-network's native XDC has a slip44 conflict upstream (it collides
with another chain's coin_type), so codefi-v2.ts addresses it via
erc20:0x0000...0000 rather than a slip44 id. Adding it here under
that same convention so the native asset isn't left without an icon.
This is a stopgap pending a cleaner long-term representation.

Author: maxime-oe