Fix to what location generated source maps are pointing in case of composed styles

[Andarist]

Currently we get first encountered source maps, but actually more interesting is the last one. Taking first can lead to some unexpected results - like in this example background-color: red styles would point to background-color: green; and it could be even defined in a completely different file.

const StyledText = styled.div`
  @media (min-width: 1040px) {
    ${css`
      background-color: green;
    `}
  }

  background-color: red;
`;

I have no idea right now what kind of a test we should write for this and I'm not sure if it's worth it, so I've skipped it for now.

This actually would also, by accident - sort of, help to workaround the stylis bug: thysultan/stylis#154

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[codecov]

Codecov Report

Merging #1653 into next will not change coverage.
The diff coverage is 100%.

Impacted Files	Coverage Δ
packages/serialize/src/index.js	100% <100%> (ø)	⬆️