Permit sending to zero address for burn

Currently, the Metamask UI (both the old and the new) does not accept `0x0000000000000000000000000000000000000000` as a valid destination address for ERC20 token transfers.

A user should be able to send a transfer to that address as an explicit "burn" operation. A warning in the UI would be nice, but there are scenarios where it is desirable to explicitly burn tokens, and transferring to the zero address seems to be the canonical way to do that.

The risk of a user accidentally inputing exactly forty 0s seems small enough that this wouldn't be unsafe.

Start by locating the MetaMask UI validation for ERC20 destination addresses in both the old and new transfer flows, then reproduce the rejection of the zero address. Confirm the expected behavior and any warning requirements with maintainers; done means an explicit transfer to 0x0000000000000000000000000000000000000000 is accepted, with a warning if agreed.

Written by the indexing model from the issue text.