fix misaligned line number in sourcemap (#3906)

[nomnomnomnom]

Fixes issue #3906. Sourcemaps line numbers for every <script> start at line 1, even when the script is lower in the file. E.g.

<!-- Some comment
and more
and more details -->
<script>
  ...
</script>

and

<script context="module">
  // correct sourcemap
  ...
</script>
<script>
  // incorrect sourcemap
  ...
</script>

The two examples above are (approximately) the two tests included in this PR. Both tests fail prior to applying the fix and pass afterwards.