Autofix generates invalid code with nested style blocks

Test case 1:

```tsx
const Div = styled.div`
${css`
color: blue;
`}
`;
```

This code yields the error `Skipping template… as it included either invalid syntax or complex expressions the plugin could not interpret`, as described at #7.

Test case 2:

```tsx
const Div = styled.div`
${css`
color: blue;
`}

div {
position: absolute;
}
`;
```

This yields the error `Unexpected unknown type selector "POSTCSS_styled-components_0" (selector-type-no-unknown)` on the `${}` line. But what's really strange is what happens if you run stylelint with `--fix`. The output is this invalid code:

```tsx
const Div = styled.div`
${css`
color: blue;
`}

div {
position: absolute;
}
color: blue;
`}

div {
position: absolute;
}
`;
```