Fix formatting of multi-line strings

[bentsherman]

Multi-line strings are not formatted correctly when changing the number of indent spaces:

    // before (indent 4 spaces)
    """
    echo '${x} world!'
    """

  // after (indent 2 spaces)
  """
    echo '${x} world!'
    """

This is because the indent is part of the string text and the formatter simply emits the string text verbatim.

Instead, the formatter should strip and re-indent the string.