Flag without an alias is being reported as unknown

```js
import mri from "mri";

const parsed = mri(process.argv.slice(2), {
string: ["foo", "bar"],
alias: { f: "foo" },
unknown(flag) {
console.error(`Unknown flag: ${flag}`);
process.exit(1);
},
});

console.log(parsed);
```

Then run the following command:

```sh
node repro.js --bar
```

## Expected

Log the parsed flags and exit without error.

## Actual

Logs `Unknown flag: --bar` and exits with code 1.

Start by running the issue's repro with `node repro.js --bar`, then trace mri's flag parsing and the path that invokes the `unknown` callback. The work is done when `--bar` is accepted as declared in `string`, the parsed flags are logged, and the process exits without error.

Written by the indexing model from the issue text.