Update template to follow best practices in namespace use (Issue #1823)

[MuyuanMS]

Addressing issue #1823 and update the template on the namespace, in order to address compiling error when projects that uses C++/WinRT along with C++/CX and/or WRL

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[MuyuanMS]

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[Scottj1s]

nesting the Microsoft::UI::Xaml under winrt explicitly is a good change to remove ambiguity.
but removing the "using namespace winrt;" will just require most users to go add it back.

as this comment notes,
"using the winrt namespace is fine as long as the project only uses C++/WinRT and doesn't use C++/CX and/or WRL along with it."

that's the default for most projects (not combining with CX and/or WRL), so we should continue optimizing for that

[codendone]

⁠I have a couple of times hit conflicts with "IInspectable" due to not specifying the namespace. But maybe that isn't the normal experience.

I agree the C++/WinRT along with C++/CX is not a scenario for majority customers.

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In reply to: 2458215921