tdl.format() does not escape quotes in docstrings

[goodmami]

Consider how the following renders invalid TDL:

>>> print(tdl.format(
...     tdl.TypeDefinition(
...         tdl.TypeIdentifier('a'),
...         tdl.TypeIdentifier('b'),
...         docstring='inner-triple-quote""" final quotes"')))
a := b
  """
  inner-triple-quote""" final quotes"
  """.

As long as we always format docstrings with newlines after the docstring contents, one or two final quotes are fine, but if the newline and spacing is dropped they will need to be escaped. Triple-quotes inside the docstring always need to be escaped (at least one of the quotes does).