TypeDefinition.IsNested Property (dotnet/corefx#25963)
* Adding API TypeDefinition.IsNested. Closes dotnet/corefx#5377
* Adding related tests.
* fixing backward asserts.
* Switching to expression-bodied syntax.
* Adding doc comment. Switching to an assembly that actually has nested types.
* Fixing xml doc comment to align with msdn comment style.
* Update TypeDefinition.cs
Fixing bad grammar in comments.
Commit migrated from https://github.com/dotnet/corefx/commit/
af18a5fe0d4aab25d8fcb6900fcffa8c99befb60