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;
}
`;
```

Look at the parser in src/syntax.js and the transformer in src/transformer.js. The bug occurs when nested template literals with css blocks are processed. Start by reproducing the error with the given test cases, then trace how the plugin handles interpolation and nested style blocks. Check the output after autofix to see where duplication happens.

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