Jss Typescript cannot accept value of array

hi, after i upgrading jss to v 10.6.0, my code break!

It seem the typescript definition is wrong. See the TS code below:

```
const otherStyle1: JssStyle = {
background: 'red',
opacity: 5,
};
const otherStyle2: JssStyle = {
background: 'red',
opacity: 5,
};
const test = createUseStyles({
main: {
extend: otherStyle1 // FINE, otherStyle1 is acceptable JssValue
},
});
const test2 = createUseStyles({
main: {
extend: [otherStyle1, otherStyle2] // <== TYPESCRIPT ERROR HERE
},
});
```
It seem the prop value only accept string, number, another style, but **not an array**
` : `

in jss however, the `extend` is a special prop name. We should either passing a single value or an array of value,
so the TS definition should be:
` : `

to temporary workaround the problem, I just force the prop value as non array:
```
const test2 = createUseStyles({
main: {
extend: [otherStyle1, otherStyle2] as JssStyle
},
});
```

Start at the TypeScript definition used by JssStyle and the createUseStyles example in the issue. Verify that extend accepts an array of the shown style values and that the example type-checks without the cast.

Written by the indexing model from the issue text.