improve CODE_BLOCK_PATTERN for a more robust code match (#571)

* improve CODE_BLOCK_PATTERN for more robust match

* improve and add tests

* Add support for \r\n

* Updated the regex to support indented code blocks (per the Markdown spec). Added test cases for both.

* Update formatting

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Co-authored-by: Adam Fourney <adamfo@microsoft.com>
Co-authored-by: Chi Wang <wang.chi@microsoft.com>