Fix conflicting definition of CommonMark/Node

[anthonyryan1]

CommonMark/Node was declared as final, but CommonMark/Node/* all extend it, leading to conflicts between the class definition and reality.

[coveralls]

Pull Request Test Coverage Report for Build 630

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 96.0%

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Totals
Change from base Build 243:	-0.2%
Covered Lines:	1896
Relevant Lines:	1975

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💛 - Coveralls