Rename requiredPrefund to missingAccountFunds

[nlordell]

This PR applies a minor suggestion resulting from the audit where the validateUserOp parameter names (specifically for the required account fund transfer to the entrypoint) did not exactly match the name from the ERC.

Note that the test entrypoint was slightly adjusted to more closely model the behaviour of the reference entrypoint, where left-over user balances are considered when computing the missingAccountFunds to pass into the validateUserOp call:

            uint256 missingAccountFunds = 0;
            if (paymaster == address(0)) {
                uint256 bal = balanceOf(sender);
                missingAccountFunds = bal > requiredPrefund
                    ? 0
                    : requiredPrefund - bal;
            }

I noticed this while looking for instances of requiredPrefund in the codebase. All tests continue to pass 🎉.

Another minor change, I noticed that prettier . was not picking up .sol files anymore, so I adjusted the NPM scripts to explicitly format and check them.

[coveralls]

Pull Request Test Coverage Report for Build 6721166899

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 6720614399:	0.0%
Covered Lines:	26
Relevant Lines:	26

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💛 - Coveralls