fix: coffeescript sourcemap

[bluwy]

Fixes #413

The coffeescript transformer was using an old sourcemap format I believe. It was something like:

SourceMap {
  lines: [
    LineMap { line: 0, columns: [Array] },
    <1 empty item>,
    LineMap { line: 2, columns: [Array] }
  ]
}

This change uses the v3SourceMap string from the compile result instead.

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Tests

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[kaisermann]

Thanks! Released in v4.9.6 🎉