Add tests to clarify heredoc behavior

[habnabit]

Some of these tests fail already, which is most certainly a mistake, because having a blank line shouldn't count as an un-indented line.

I don't know if the expected results are all correct, but it's better to codify this behavior as regression tests anyway, since it's an important and missing case.

I can edit the branch to whatever should be considered correct, but opening this PR is at least in part asking "what should be considered correct?"

[hashicorp-cla]

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