Introduce static AssetLoader

[maxkatz6]

What does the pull request do?

1. Rename instance-class impl AssetLoader to StandardAssetLoader and make it internal.
2. Add public and static AssetLoader with supported APIs
3. Update assetloader tests to use StandardAssetLoader properly without static global state.
4. Update IAssetLoader obsolete message (in another PR).

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Documentation with user documentation

Breaking changes

AssetLoader class is now internal and renamed to StandardAssetLoader. Static API covers all supported use-cases.

Fixed issues

Fixes #11155

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0034047-beta. (feed url: https://pkgs.dev.azure.com/AvaloniaUI/AvaloniaUI/_packaging/avalonia-all/nuget/v3/index.json) [PRBUILDID]