ERC20 contract loses tokens sent to the contract's own address

🧐 Motivation
Many people lose their ERC20 tokens by mistakenly sending their tokens back to the token contract address (presumably to redeem their tokens). However, there is no way to recover those tokens.
https://np.reddit.com/r/ethereum/comments/sfz4kw/did_i_just_lose_half_a_million_dollars_by_sending/

📝 Details
ERC20 implementation should reject such transfers.
For example, we may add a require statement to the _transfer function in ERC20.sol.

    function _transfer(
        address sender,
        address recipient,
        uint256 amount
    ) internal virtual {
        ...
        require(recipient != address(this), "ERC20: transfer to the token contract address");
        ...
    }

Start with contracts/token/ERC20/ERC20.sol and inspect the _transfer function and its surrounding behavior. Confirm how transfers to the token contract address are currently handled, then verify that the completed change rejects that destination without affecting other ERC20 transfers.