Added solution filter for .NET projects (#3906)

## PR Type
What kind of change does this PR introduce?
<!-- Please uncomment one or more that apply to this PR. -->

- Infrastructure
<!-- - Bugfix -->
<!-- - Feature -->
<!-- - Code style update (formatting) -->
<!-- - Refactoring (no functional changes, no api changes) -->
<!-- - Build or CI related changes -->
<!-- - Documentation content changes -->
<!-- - Sample app changes -->
<!-- - Other... Please describe: -->


## What is the current behavior?
<!-- Please describe the current behavior that you are modifying, or link to a relevant issue. -->
There is no solution filter or solution available to only work on .NET-based projects. This has two issues:
- It is extremely cumbersome for developers on either other platforms (eg. Linux, iOS) or on Windows but without the necessary UWP SDKs installed to open the solution and contribute, as they'd have lots of projects failing to open, VS showing all sorts of warnings and error windows, etc.
- Even for developers with all needed SDKs, opening the entire solution every time even when just working on a .NET component takes longer to load, makes VS slower due to the higher number of loaded projects, makes build times longer, etc.

## What is the new behavior?
<!-- Describe how was this issue resolved or changed? -->
There is now a solution filter with just the .NET projects, which makes it super easy to only load up those projects to contribute to the Toolkit. The solution filter is **much** faster to load and makes it generally a better experience to work in this scenario 😄

## PR Checklist

Please check if your PR fulfills the following requirements:

- [X] Tested code with current [supported SDKs](../readme.md#supported)
- [ ] ~~Pull Request has been submitted to the documentation repository [instructions](..\contributing.md#docs). Link: <!-- docs PR link -->~~
- [ ] ~~Sample in sample app has been added / updated (for bug fixes / features)~~
    - [ ] ~~Icon has been created (if new sample) following the [Thumbnail Style Guide and templates](https://github.com/windows-toolkit/WindowsCommunityToolkit-design-assets)~~
- [ ] ~~New major technical changes in the toolkit have or will be added to the [Wiki](https://github.com/windows-toolkit/WindowsCommunityToolkit/wiki) e.g. build changes, source generators, testing infrastructure, sample creation changes, etc...~~
- [X] Tests for the changes have been added (for bug fixes / features) (if applicable)
- [X] Header has been added to all new source files (run *build/UpdateHeaders.bat*)
- [X] Contains **NO** breaking changes

Windows Community Toolkit (NET).slnf

@@ -0,0 +1,15 @@
+{
+  "solution": {
+    "path": "Windows Community Toolkit.sln",
+    "projects": [
+      "Microsoft.Toolkit.Diagnostics\\Microsoft.Toolkit.Diagnostics.csproj",
+      "Microsoft.Toolkit.HighPerformance\\Microsoft.Toolkit.HighPerformance.csproj",
+      "Microsoft.Toolkit.Mvvm\\Microsoft.Toolkit.Mvvm.csproj",
+      "Microsoft.Toolkit\\Microsoft.Toolkit.csproj",
+      "UnitTests\\UnitTests.HighPerformance.NetCore\\UnitTests.HighPerformance.NetCore.csproj",
+      "UnitTests\\UnitTests.HighPerformance.Shared\\UnitTests.HighPerformance.Shared.shproj",
+      "UnitTests\\UnitTests.NetCore\\UnitTests.NetCore.csproj",
+      "UnitTests\\UnitTests.Shared\\UnitTests.Shared.shproj"
+    ]
+  }
+}