add nillion icon and metadata

[tlitre]

Note

Adds Nillion (NIL) ERC-20 token assets to the repository.

• New metadata/eip155:1/erc20:0x7Cf9a80db3B29eE8efE3710AadB7b95270572d47.json defining name, symbol, decimals: 6, erc20: true, and logo path
• New SVG logo at icons/eip155:1/erc20:0x7Cf9a80db3B29eE8efE3710AadB7b95270572d47.svg

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