Add error when `flag.default` isn't a valid choice

[tommy-mitchell]

meow({
    flags: {
        number: {
            type: 'number',
            choices: [1, 2, 3],
            default: 1,
        },
    },
});
//=> Throws 'Each value of the option `default` must exist within the option `choices`. Invalid flags: `--number`'