Handle newlines inside an IEx prompt

[axelson]

When given text like:

iex> existing_zipper = ("""
...> IO.inspect("Hello, world!")
...> IO.puts("abc")
...> """

We need to ensure that the newlines are preserved in the selectable text. With this PR ex_doc will allow us to copy the text:

existing_zipper = ("""
IO.inspect("Hello, world!")
IO.puts("abc")
"""

Without this PR, ex_doc copies the text:

existing_zipper = ("""IO.inspect("Hello, world!")IO.puts("abc")"""

and this breaks when pasted into iex with an error like:

** (SyntaxError) invalid syntax found on iex:1:23:
    error: heredoc allows only whitespace characters followed by a new line after opening """

Fixes elixir-lang/ex_doc#2096

[josevalim]

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