Expose a library's Resource Loader, allow a user to "subset" it (#19131)

This pull request elevates ScopedResourceLoader to the API surface of
LibraryResources, which will allow any library resource consumer to
directly interact with its resource loader.

One of those new interactions is to make a sub-context with a specific
narrowed-down qualifier. Like this:

```c++
auto englishOnlyLoader = GetLibraryResourceLoader().WithQualifier(L"language", L"en-us");
/* auto foo = */ englishOnlyLoader.GetLocalizedString(USES_RESOURCE(L"AppName"));
```

Author: DHowett

src/cascadia/WinRTUtils/LibraryResources.cpp

@@ -76,10 +76,10 @@ static void EnsureAllResourcesArePresent(const ScopedResourceLoader& loader)
 
 #endif
 
-static ScopedResourceLoader GetLibraryResourceLoader()
+const ScopedResourceLoader& GetLibraryResourceLoader()
 try
 {
-    ScopedResourceLoader loader{ g_WinRTUtilsLibraryResourceScope };
+    static ScopedResourceLoader loader{ g_WinRTUtilsLibraryResourceScope };
 #ifdef _DEBUG
     EnsureAllResourcesArePresent(loader);
 #endif
@@ -90,15 +90,13 @@ CATCH_FAIL_FAST()
 winrt::hstring GetLibraryResourceString(const std::wstring_view key)
 try
 {
-    static auto loader{ GetLibraryResourceLoader() };
-    return loader.GetLocalizedString(key);
+    return GetLibraryResourceLoader().GetLocalizedString(key);
 }
 CATCH_FAIL_FAST()
 
 bool HasLibraryResourceWithName(const std::wstring_view key)
 try
 {
-    static auto loader{ GetLibraryResourceLoader() };
-    return loader.HasResourceWithName(key);
+    return GetLibraryResourceLoader().HasResourceWithName(key);
 }
 CATCH_FAIL_FAST()

src/cascadia/WinRTUtils/ScopedResourceLoader.cpp

@@ -46,3 +46,14 @@ bool ScopedResourceLoader::HasResourceWithName(const std::wstring_view resourceN
 {
     return _resourceMap.HasKey(resourceName);
 }
+
+ScopedResourceLoader::ScopedResourceLoader(winrt::Windows::ApplicationModel::Resources::Core::ResourceMap map, winrt::Windows::ApplicationModel::Resources::Core::ResourceContext context) noexcept :
+    _resourceMap{ std::move(map) }, _resourceContext{ std::move(context) } {}
+
+ScopedResourceLoader ScopedResourceLoader::WithQualifier(const wil::zwstring_view qualifierName, const wil::zwstring_view qualifierValue) const
+{
+    auto newContext = _resourceContext.Clone();
+    auto qualifierValues = newContext.QualifierValues();
+    qualifierValues.Insert(qualifierName, qualifierValue); // mutable!
+    return ScopedResourceLoader{ _resourceMap, std::move(newContext) };
+}

src/cascadia/WinRTUtils/inc/LibraryResources.h

@@ -67,6 +67,9 @@ namespace Microsoft::Console::Utils
     __pragma(warning(suppress : 26485));       \
     __declspec(selectany) extern const wchar_t* g_WinRTUtilsLibraryResourceScope{ (x) };
 
+class ScopedResourceLoader;
+
+const ScopedResourceLoader& GetLibraryResourceLoader();
 winrt::hstring GetLibraryResourceString(const std::wstring_view key);
 bool HasLibraryResourceWithName(const std::wstring_view key);
 

src/cascadia/WinRTUtils/inc/ScopedResourceLoader.h

@@ -11,7 +11,10 @@ class ScopedResourceLoader
     winrt::hstring GetLocalizedString(const std::wstring_view resourceName) const;
     bool HasResourceWithName(const std::wstring_view resourceName) const;
 
+    ScopedResourceLoader WithQualifier(const wil::zwstring_view qualifierName, const wil::zwstring_view qualifierValue) const;
+
 private:
+    ScopedResourceLoader(winrt::Windows::ApplicationModel::Resources::Core::ResourceMap map, winrt::Windows::ApplicationModel::Resources::Core::ResourceContext context) noexcept;
     winrt::Windows::ApplicationModel::Resources::Core::ResourceMap _resourceMap;
     winrt::Windows::ApplicationModel::Resources::Core::ResourceContext _resourceContext;
 };