Unpackaged Win32 app using WinUI3 Xaml Island - problems with external fonts referenced in xaml

Nobody has claimed this yet.

### Describe the bug

`` does not work.

It is probably a very old problem that no one tried to fix or find a workaround. It was [reported](https://github.com/microsoft/microsoft-ui-xaml/issues/3766) almost five years ago...

WinUI3 Xaml does not support `file:` as part of `Uri`. It can be seen in `ResourceManager::TryGetLocalResource` where only `ms-appdata:`, `ms-appx:` and `ms-resource:` are handled.

That is understandable for UWP applicarions and Packaged Desktop applications (although when they have `broadFileSystemAccess` it should work too, right?). But it should work for Unpackaged Deskop application.

While searching for workaround I found that

- `ms-appdata:` cannot be used because it requires packaged application.
- `ms-appx:` and `ms-resource:` can be used in unpackaged application to reach files in exe directory (and its subdirectories)

`ms-appx:///xxx` is essentialy transformed to `ms-resource:///files/xxx`.
`ms-resource:///files/xxx` uses `m_pFallbackResourceProvider` and combines `xxx `with `m_pBaseUri`. `m_pBaseUri` is initialized to `GetModuleFileName(NULL, applicationDirectory, MAX_PATH)` in `CommonResourceProvider::Create`.

This workaround looks like implementation detail and might stop working in future. And of course is limited to exe directory only. What else should we use?

### Steps to reproduce the bug

`` does not work.

### Expected behavior

`` Should work.

### Screenshots

_No response_

### NuGet package version

WinUI 3 - Windows App SDK 1.6.1: 1.6.240923002

### Windows version

Windows 11 (22H2): Build 22621

### Additional context

_No response_

Start by reading ResourceManager::TryGetLocalResource and CommonResourceProvider::Create, focusing on how file:, ms-appdata:, ms-appx:, and ms-resource: URIs are handled. Reproduce the external-font case in an unpackaged Win32 Xaml Island application and determine how the resource provider should resolve a font outside the executable directory. Done means the file:///c:/font.ttf#font reference works as expected.

Written by the indexing model from the issue text.