parsing of strings containing flag notation

```js
#!/usr/bin/env node
const mri = require('mri')
console.log(process.argv)
console.log(mri(process.argv.slice(2)))
```

```shell
./args.js -a -b '-c foo'
```

```js
[
'/usr/local/Cellar/node/16.0.0_1/bin/node',
'/Users/j/web/pev2-cli/foo.js',
'-a',
'-b',
'-c foo'
]
{
_: [],
a: true,
b: true,
c: true,
' ': true,
f: true,
o: [ true, true ]
}
```

Is that intended?

I would have expected this:

```js
{
_: ['-c foo'],
a: true,
b: true
}
```

Or maybe this (but I find the result above much more intuitive):

```js
{
_: [],
a: true,
b: true,
c: 'foo'
}
```

Reproduce the reported case with the mri entry point and the shown process.argv.slice(2) arguments. Inspect how grouped flags and quoted values are parsed, then confirm the intended result with the project maintainers before defining tests or a completion criterion.

Written by the indexing model from the issue text.