Make AllowNull internal to prevent conflicts

[mkapahnke]

• Unity uses a version of "netstandard2.1", but already includes the AllowNull attribute
• This will prevent conflicts with other packages, that might include this attribute as well

[mkapahnke]

I guess, that this should not have an affect on other Markdig code.

The only problem would be, if someone uses the Markdig AllowNull.

[coveralls]

Pull Request Test Coverage Report for Build 5659390425

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 93.323%

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Totals
Change from base Build 4772706458:	0.0%
Covered Lines:	25978
Relevant Lines:	27233

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💛 - Coveralls