Avoid treating /r as newline on it (dart-lang/string_scanner#81)

Fixes https://github.com/dart-lang/string_scanner/issues/80

This fixes an error when scanning a zero-length match when between a CR and LF. 


* Fix typo

* comment nits

* Fix some bugs when setting position if the current position is between \r\n

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Co-authored-by: Nate Bosch <nbosch1@gmail.com>