Cannot use function values or function rules in TypeScript

__Expected behavior:__
Function values and function rules should render the appropriate CSS.

__Describe the bug:__
The rule which includes a function does not render. This is the typescript error (for function rules)

```
Index signature is missing in type '(data: any) => { backgroundColor: any; width: number; height: number; }'
```

__Codesandbox link:__
Please create a [codesandbox.io](https://codesandbox.io/) with the issue. Make it as minimal as possible as this will help us find the bug quicker.

https://codesandbox.io/s/nifty-visvesvaraya-7eq2v?file=/src/index.ts

__Versions (please complete the following information):__
- jss: 10.1.1
- jss-plugin-camel-case: 10.1.1
- Browser [e.g. chrome, safari]:
- OS [e.g. Windows, macOS]: Windows 10 (Also present in CodeSandbox)
Feel free to add any additional versions which you may think are relevant to the bug.