Reduce the content exported by Namespace.0.h

[YexuanXiao]

No description provided.

[Copilot]

Pull request overview

This PR adjusts the code generation for Namespace.0.h to reduce what gets exported from the winrt::impl namespace (primarily relevant for C++20 module builds where WINRT_EXPORT expands to export extern "C++").

Changes:

• Added a wrap_impl_namespace_without_export helper to emit namespace winrt::impl { ... } without WINRT_EXPORT.
• Updated write_namespace_0_h generation to use the non-exporting winrt::impl wrapper for categories/names/guids and most ABI-related output.
• Split struct ABI generation into two passes (write_struct_abi_type and write_struct_abi_specialization) so only the struct ABI type emission remains under the exporting wrapper.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
cppwinrt/file_writers.h	Updates Namespace.0.h emission to selectively avoid WINRT_EXPORT and splits struct ABI output into separate wrapped blocks.
cppwinrt/code_writers.h	Adds wrap_impl_namespace_without_export and splits struct ABI writer into type vs abi<> specialization emitters.