Use SlnGen to generate solution from Traversal project (#5600)

Updates repository to use `Microsoft.Build.Traversal` `.proj` files to
drive which projects are built. Previously the arcade build
infrastructure would build the solution files found in the root
directly. This approach gives us more fine-grained control over projects
to build, as well as allow us to automatically generate a solution file
using `slngen`.

I have checked in the generated `build.sln` which can be loaded with
`VS` or `VSCode`. This version includes the `vcxproj` files which
improve the native debug experience in VS. There is an option to
generate `build.sln` without these files as they can cause warnings on
non-Windows platforms (same behavior as today).

For convienence I have added two new scripts:
* `eng/generate-sln.ps1`
* `eng/generate-sln.sh`
both of these automatically invoke the proper commands to regenerate the
`build.sln` file.

I updated the instruction docs to reference `build.sln` and explain how
to regenerate it.

Edit:
* I have reverted from using `slnx -> sln` as the conversion process
causes some problems with the vcxproj Ids. Given that these are now
auto-generated, using the `sln` format seems acceptable as it has
broader support.

build.proj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.Build.Traversal">
+
+  <ItemGroup >
+    <ProjectReference Include="src/dirs.proj" />
+    <ProjectReference Include="src/tests/dirs.proj" Condition="'$(SkipTests)' != 'true'" />
+  </ItemGroup>
+
+</Project>