refactor: Extract children property into interface

[nzakas]

This PR extracts the common children property into helper interfaces.

• CssParentNodeCommon - for regular nodes that always have children
• CssParentNodeCommonPlain - for plain nodes that always have children
• CssOptionalParentNodeCommon - for regular nodes that sometimes have children
• CssOptionalParentNodeCommonPlain - for plain nodes that always have children

This makes it easier when people are defining custom node types to quickly ensure their nodes are implementing children the correct way.

No test updates because this is a structural refactor so it doesn't change how the types work, just how they are organized.

[coveralls]

Pull Request Test Coverage Report for Build 15536377430

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 98.611%

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Totals
Change from base Build 15471406727:	0.0%
Covered Lines:	11823
Relevant Lines:	11955

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💛 - Coveralls

[fasttime]

Just a suggestion, otherwise LGTM.

[fasttime]

LGTM, thanks!