fix(zap_dev): Wrap interpolated identifiers in braces

[dnys1]

When component variables are interpolated in strings, the replacement identifier can cause invalidate Dart code to be generated if the identifier is not wrapped in braces already.

For example:

<script>
  String? value;
  String? interpolated;

  $: interpolated = '$value';
</script>

Here, the interpolated string currently becomes something like $_$v2 /* value */ which is invalid Dart code, when we really want ${_$v2 /* value */}.