Solution filter (`*.slnf`) support

Is your feature request related to a problem? Please describe.

When opening a large solution in Stride Game Studio, all projects are loaded, including unit test projects and other non-Stride executables. This results in unnecessary load times and increased memory usage. Additionally, non-game projects (e.g., unit test projects) are often incorrectly set as startup projects, which leads to incorrect visual indicators in the asset view.

In larger solutions (in my case with about 50 projects), manually selecting the correct startup project each time is disruptive and inefficient.

Describe the solution you'd like

Stride Game Studio should support Solution Filter Files (*.slnf), enabling users to open only the necessary projects as a subset of the complete solution. This would improve load performance and prevent unwanted projects (e.g., unit test projects) from being included in the game editor.

The solution filter files are created in Visual Studio and can be opened with it. When opening a slnf-file, it is easy to update the included projects. The filter file has a reference to a solution file (sln) to achieve this.

Expected behavior:

• Stride Game Studio should support opening .slnf files just like .sln files.
• Only the projects specified in the .slnf file should be loaded, reducing memory usage and load time.

Describe alternatives you've considered

• Manually setting the startup project: This is possible but tedious, especially when frequently switching between game studio and Visual Studio or often restarts of the game studio to refresh scripts.
• Using separate solution files (.sln): A workaround is to create a secondary .sln file containing only the relevant projects. However, maintaining multiple solution files is cumbersome, as any new project additions must be manually synchronized.

Additional context

Use Cases

• Developers working on large solutions containing both game-related and non-game-related projects.
• Teams using unit tests that should not be loaded in Stride Game Studio.
• Optimizing memory usage and load performance by excluding unnecessary projects.
• Selecting the correct startup project when opening the game studio. Therefore, it is necessary only have one executable project loaded.

Editor Behavior

• The editor should allow .slnf files to be opened just like .sln files.
• When opening a .slnf file, only the projects listed in it should be loaded.

Components Involved

• Project loading mechanism in Stride Game Studio. Especially the Stride.Core.Assets project.
• Launcher must list the last opened sln and slnf-files.

Start by tracing the project loading mechanism in Stride.Core.Assets and how the launcher handles recently opened solution files. Compare the existing .sln flow with the .slnf requirements in the issue. Done means the editor opens .slnf files, loads only their listed projects, and the launcher retains both .sln and .slnf entries.